[HttpKernel] Fixed RequestDataCollector handling of null header values.

Author: Gabriel Moreira

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);

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'));