minor #21001 [HttpKernel] Continuation of #20569 (ro0NL)

This PR was merged into the 3.3-dev branch.

Discussion
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[HttpKernel] Continuation of #20569

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/pull/20567#discussion_r92928110
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!--highly recommended for new features-->

Forgotten in #20569 and makes #20567 truly a feature only.

Before

![image](https://cloud.githubusercontent.com/assets/1047696/21361738/971e4f8e-c6e5-11e6-8b08-8a5f8ab4f1f0.png)

After

![image](https://cloud.githubusercontent.com/assets/1047696/21361752/a6dc5380-c6e5-11e6-9f18-3bc3d7db94be.png)

Commits
-------

e1616b3 [HttpKernel] Continuation of #20569

src/Symfony/Component/HttpKernel/DataCollector/RequestDataCollector.php

@@ -39,11 +39,6 @@ class RequestDataCollector extends DataCollector implements EventSubscriberInter
      */
     public function collect(Request $request, Response $response, \Exception $exception = null)
     {
-        $responseHeaders = $response->headers->all();
-        foreach ($response->headers->getCookies() as $cookie) {
-            $responseHeaders['set-cookie'][] = (string) $cookie;
-        }
-
         // attributes are serialized and as they can be anything, they need to be converted to strings.
         $attributes = array();
         $route = '';
@@ -95,7 +90,7 @@ class RequestDataCollector extends DataCollector implements EventSubscriberInter
             'request_cookies' => $request->cookies->all(),
             'request_attributes' => $attributes,
             'route' => $route,
-            'response_headers' => $responseHeaders,
+            'response_headers' => $response->headers->all(),
             'session_metadata' => $sessionMetadata,
             'session_attributes' => $sessionAttributes,
             'flashes' => $flashes,