Update http_client.rst

ThomasLandauer · web-flow · commit c8e5dd6aa9d2 · 2020-09-13T17:02:25.000+02:00
Integrating the findings of symfony/symfony#38082 (comment)
diff --git a/http_client.rst b/http_client.rst
@@ -642,6 +642,11 @@ when the streams are large)::
         'body' => $formData->bodyToString(),
     ]);
 
+If you need to add a custom HTTP header to the upload, you can do::
+
+    $headers = $formData->getPreparedHeaders()->toArray();
+    $headers[] = 'X-Foo: bar';
+
 Cookies
 ~~~~~~~
 
