merged branch fabpot/format-fix (PR #8961)

This PR was merged into the 2.2 branch.

Discussion
----------

fixed regression in the way the request format is handled

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8917
| License       | MIT
| Doc PR        | n/a

Commits
-------

719b2bf [HttpFoundation] fixed regression in the way the request format is handled for duplicated requests (closes #8917)

src/Symfony/Component/HttpFoundation/Request.php

@@ -407,7 +407,8 @@ class Request
         $dup->format = null;
 
         if (!$dup->get('_format')) {
-            $dup->setRequestFormat($this->getRequestFormat());
+            // we set the request format to null if the current request is not known
+            $dup->setRequestFormat($this->getRequestFormat(null));
         }
 
         return $dup;