"[HttpFoundation] fixed Request::create() method" breaks BrowserKit Client

To functional test a REST service I call 'Symfony\Bundle\FrameworkBundle\Client::request' with the 'server' parameter. This parameter contains:
```
[HTTP_CONTENT-TYPE] => application/json
[HTTP_ACCEPT] => application/json
```

Since latest master (this PR in particular: https://github.com/symfony/symfony/pull/6995) it fails my functional tests. I suspect the problem is somewhere near this piece of code (line: 320|329):
```
-    $defaults['CONTENT_TYPE'] = 'application/x-www-form-urlencoded';
+    $server['CONTENT_TYPE'] = 'application/x-www-form-urlencoded';
```