This PR was merged into the 2.8 branch.

Discussion
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[BrowserKit] fixed BC Break for HTTP_HOST header

| Q             | A
| ------------- | ---
| Branch?       | 2.7, 2.8, 3.x, 4.x
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes/no
| Fixed tickets | #22933
| License       | MIT
| Doc PR        | n/a

The situation well described in the original issue. I will add only:

- #10549 - makes server parameters to take precedence over URI.
- #16265 - partially revererts  #10549. Makes server parameters do not affect URI. But this is only true for `Client::request()`. It is still possible to set host for URI by `Client::setServerParameters()` when URI is realative (see examples below).

I propose a compromise solution: add to HTTP_HOST header power to override URI when it is relative.

Proposed solution:
- if the request URI is relative, then use the HTTP_HOST header passed to Client::request() to generate an absolute URI
- if the request URI is absolute, then ignore the HTTP_HOST header (as it now works)
- do the same with HTTPS server parameter

Profit:
- fix BC Break
- the documentation will be correct
  - http://symfony.com/doc/2.8/routing/hostname_pattern.html#testing-your-controllers
  - https://symfony.com/doc/2.8/testing.html#testing-configuration

Before:

```
$client->setServerParameters(['HTTP_HOST' => 'example.com']);
$client->request('GET', '/');
$this->assertEquals('http://example.com/', $client->getRequest()->getUri());

$client->request('GET', '/', [], [], ['HTTP_HOST' => 'example.com']);
$this->assertEquals('http://localhost/', $client->getRequest()->getUri());
```

Fixed (see last line):

```
$client->setServerParameters(['HTTP_HOST' => 'example.com']);
$client->request('GET', '/');
$this->assertEquals('http://example.com/', $client->getRequest()->getUri());

$client->request('GET', '/', [], [], ['HTTP_HOST' => 'example.com']);
$this->assertEquals('http://example.com/', $client->getRequest()->getUri());
```

Commits
-------

8c4a594 [BrowserKit] fixed BC Break for HTTP_HOST header; implemented same behaviour for HTTPS server parameter