[HttpFoundation] fixed a bug in previous merge

src/Symfony/Component/HttpFoundation/Response.php

@@ -167,7 +167,7 @@ class Response
      */
     public function setContent($content)
     {
-        if (!is_string($content) && !is_numeric($content) && !is_callable(array($content, '__toString'))) {
+        if (null !== $content && !is_string($content) && !is_numeric($content) && !is_callable(array($content, '__toString'))) {
             throw new \UnexpectedValueException('The Response content must be a string or object implementing __toString(), "'.gettype($content).'" given.');
         }