diff --git a/RedirectResponse.php b/RedirectResponse.php
index 13da56a75..c6d7590ef 100644
--- a/RedirectResponse.php
+++ b/RedirectResponse.php
@@ -42,7 +42,7 @@ public function __construct(string $url, int $status = 302, array $headers = [])
             throw new \InvalidArgumentException(sprintf('The HTTP status code is not a redirect ("%s" given).', $status));
         }
 
-        if (301 == $status && !\array_key_exists('cache-control', array_change_key_case($headers, \CASE_LOWER))) {
+        if (301 == $status && !\array_key_exists('cache-control', array_change_key_case($headers, CASE_LOWER))) {
             $this->headers->remove('cache-control');
         }
     }
diff --git a/Response.php b/Response.php
index 895d110f4..4a331954b 100644
--- a/Response.php
+++ b/Response.php
@@ -258,10 +258,12 @@ public function prepare(Request $request)
             $this->setContent(null);
             $headers->remove('Content-Type');
             $headers->remove('Content-Length');
+            // prevent PHP from sending the Content-Type header based on default_mimetype
+            ini_set('default_mimetype', '');
         } else {
             // Content-type based on the Request
             if (!$headers->has('Content-Type')) {
-                $format = $request->getPreferredFormat();
+                $format = $request->getPreferredFormat(null);
                 if (null !== $format && $mimeType = $request->getMimeType($format)) {
                     $headers->set('Content-Type', $mimeType);
                 }
diff --git a/Session/Storage/Handler/AbstractSessionHandler.php b/Session/Storage/Handler/AbstractSessionHandler.php
index 0618035b4..bd151669c 100644
--- a/Session/Storage/Handler/AbstractSessionHandler.php
+++ b/Session/Storage/Handler/AbstractSessionHandler.php
@@ -114,7 +114,7 @@ public function destroy($sessionId)
     {
         if (!headers_sent() && filter_var(ini_get('session.use_cookies'), FILTER_VALIDATE_BOOLEAN)) {
             if (!$this->sessionName) {
-                throw new \LogicException(sprintf('Session name cannot be empty, did you forget to call "parent::open()" in "%s"?.', \get_class($this)));
+                throw new \LogicException(sprintf('Session name cannot be empty, did you forget to call "parent::open()" in "%s"?.', static::class));
             }
             $cookie = SessionUtils::popSessionCookie($this->sessionName, $sessionId);
 
diff --git a/Session/Storage/Handler/SessionHandlerFactory.php b/Session/Storage/Handler/SessionHandlerFactory.php
index 1f017c8af..b92bd0ffd 100644
--- a/Session/Storage/Handler/SessionHandlerFactory.php
+++ b/Session/Storage/Handler/SessionHandlerFactory.php
@@ -54,7 +54,7 @@ public static function createHandler($connection): AbstractSessionHandler
             case 0 === strpos($connection, 'rediss:'):
             case 0 === strpos($connection, 'memcached:'):
                 if (!class_exists(AbstractAdapter::class)) {
-                    throw new InvalidArgumentException(sprintf('Unsupported DSN "%s". Try running "composer require symfony/cache".', $connection));
+                    throw new \InvalidArgumentException(sprintf('Unsupported DSN "%s". Try running "composer require symfony/cache".', $connection));
                 }
                 $handlerClass = 0 === strpos($connection, 'memcached:') ? MemcachedSessionHandler::class : RedisSessionHandler::class;
                 $connection = AbstractAdapter::createConnection($connection, ['lazy' => true]);
diff --git a/Session/Storage/NativeSessionStorage.php b/Session/Storage/NativeSessionStorage.php
index eb754dbbf..792bacf4e 100644
--- a/Session/Storage/NativeSessionStorage.php
+++ b/Session/Storage/NativeSessionStorage.php
@@ -139,7 +139,7 @@ public function start()
             return true;
         }
 
-        if (\PHP_SESSION_ACTIVE === session_status()) {
+        if (PHP_SESSION_ACTIVE === session_status()) {
             throw new \RuntimeException('Failed to start the session: already started by PHP.');
         }
 
@@ -202,7 +202,7 @@ public function setName(string $name)
     public function regenerate(bool $destroy = false, int $lifetime = null)
     {
         // Cannot regenerate the session ID for non-active sessions.
-        if (\PHP_SESSION_ACTIVE !== session_status()) {
+        if (PHP_SESSION_ACTIVE !== session_status()) {
             return false;
         }
 
@@ -362,7 +362,7 @@ public function isStarted()
      */
     public function setOptions(array $options)
     {
-        if (headers_sent() || \PHP_SESSION_ACTIVE === session_status()) {
+        if (headers_sent() || PHP_SESSION_ACTIVE === session_status()) {
             return;
         }
 
@@ -427,7 +427,7 @@ public function setSaveHandler($saveHandler = null)
         }
         $this->saveHandler = $saveHandler;
 
-        if (headers_sent() || \PHP_SESSION_ACTIVE === session_status()) {
+        if (headers_sent() || PHP_SESSION_ACTIVE === session_status()) {
             return;
         }
 
diff --git a/Session/Storage/Proxy/AbstractProxy.php b/Session/Storage/Proxy/AbstractProxy.php
index ded79d64a..38f7dede6 100644
--- a/Session/Storage/Proxy/AbstractProxy.php
+++ b/Session/Storage/Proxy/AbstractProxy.php
@@ -65,7 +65,7 @@ public function isWrapper()
      */
     public function isActive()
     {
-        return \PHP_SESSION_ACTIVE === session_status();
+        return PHP_SESSION_ACTIVE === session_status();
     }
 
     /**
diff --git a/Tests/ResponseTest.php b/Tests/ResponseTest.php
index 8139dccee..6e6a210ee 100644
--- a/Tests/ResponseTest.php
+++ b/Tests/ResponseTest.php
@@ -461,18 +461,10 @@ public function testSetVary()
 
     public function testDefaultContentType()
     {
-        $headerMock = $this->getMockBuilder('Symfony\Component\HttpFoundation\ResponseHeaderBag')->setMethods(['set'])->getMock();
-        $headerMock->expects($this->at(0))
-            ->method('set')
-            ->with('Content-Type', 'text/html');
-        $headerMock->expects($this->at(1))
-            ->method('set')
-            ->with('Content-Type', 'text/html; charset=UTF-8');
-
         $response = new Response('foo');
-        $response->headers = $headerMock;
-
         $response->prepare(new Request());
+
+        $this->assertSame('text/html; charset=UTF-8', $response->headers->get('Content-Type'));
     }
 
     public function testContentTypeCharset()
diff --git a/Tests/Session/Storage/PhpBridgeSessionStorageTest.php b/Tests/Session/Storage/PhpBridgeSessionStorageTest.php
index 206ff4877..b4fad7688 100644
--- a/Tests/Session/Storage/PhpBridgeSessionStorageTest.php
+++ b/Tests/Session/Storage/PhpBridgeSessionStorageTest.php
@@ -61,12 +61,12 @@ public function testPhpSession()
     {
         $storage = $this->getStorage();
 
-        $this->assertNotSame(\PHP_SESSION_ACTIVE, session_status());
+        $this->assertNotSame(PHP_SESSION_ACTIVE, session_status());
         $this->assertFalse($storage->isStarted());
 
         session_start();
         $this->assertTrue(isset($_SESSION));
-        $this->assertSame(\PHP_SESSION_ACTIVE, session_status());
+        $this->assertSame(PHP_SESSION_ACTIVE, session_status());
         // PHP session might have started, but the storage driver has not, so false is correct here
         $this->assertFalse($storage->isStarted());