minor symfony#13629 RequestDataCollector - small fix (ovr)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes symfony#13629).

Discussion
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RequestDataCollector - small fix

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Commits
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7ed48db RequestDataCollector - small fix

Author: fabpot

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

@@ -88,7 +88,7 @@ public function collect(Request $request, Response $response, \Exception $except
         $this->data = array(
             'format' => $request->getRequestFormat(),
             'content' => $content,
-            'content_type' => $response->headers->get('Content-Type') ? $response->headers->get('Content-Type') : 'text/html',
+            'content_type' => $response->headers->get('Content-Type', 'text/html'),
             'status_text' => isset(Response::$statusTexts[$statusCode]) ? Response::$statusTexts[$statusCode] : '',
             'status_code' => $statusCode,
             'request_query' => $request->query->all(),