bug symfony#20747 [HttpKernel] Fixed RequestDataCollector handling of null header values. (Gabriel Moreira)

This PR was submitted for the master branch but it was merged into the 3.2 branch instead (closes symfony#20747).

Discussion
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[HttpKernel] Fixed RequestDataCollector handling of null header values.

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This PR references this [discussion](symfony/http-kernel@1fee784#commitcomment-20028839).

Commits
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adc4a26 [HttpKernel] Fixed RequestDataCollector handling of null header values.

Author: fabpot

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

@@ -125,7 +125,7 @@ public function collect(Request $request, Response $response, \Exception $except
                 continue;
             }
             if ('request_headers' === $key || 'response_headers' === $key) {
-                $value = array_map(function ($v) { return isset($v[1]) ? $v : $v[0]; }, $value);
+                $value = array_map(function ($v) { return isset($v[0]) && !isset($v[1]) ? $v[0] : $v; }, $value);
             }
             if ('request_server' !== $key && 'request_cookies' !== $key) {
                 $this->data[$key] = array_map(array($this, 'cloneVar'), $value);

src/Symfony/Component/HttpKernel/Tests/DataCollector/RequestDataCollectorTest.php

@@ -234,6 +234,7 @@ protected function createResponse()
         $response = new Response();
         $response->setStatusCode(200);
         $response->headers->set('Content-Type', 'application/json');
+        $response->headers->set('X-Foo-Bar', null);
         $response->headers->setCookie(new Cookie('foo', 'bar', 1, '/foo', 'localhost', true, true));
         $response->headers->setCookie(new Cookie('bar', 'foo', new \DateTime('@946684800')));
         $response->headers->setCookie(new Cookie('bazz', 'foo', '2000-12-12'));