[HttpFoundation] fixed StreamedResponse with HEAD as a method (closes symfony#4502)

Author: fabpot

src/Symfony/Component/HttpFoundation/Response.php

@@ -206,7 +206,7 @@ public function prepare(Request $request)
         $headers = $this->headers;
 
         if ($this->isInformational() || in_array($this->statusCode, array(204, 304))) {
-            $this->setContent('');
+            $this->setContent(null);
         }
 
         // Content-type based on the Request
@@ -234,7 +234,7 @@ public function prepare(Request $request)
         if ('HEAD' === $request->getMethod()) {
             // cf. RFC2616 14.13
             $length = $headers->get('Content-Length');
-            $this->setContent('');
+            $this->setContent(null);
             if ($length) {
                 $headers->set('Content-Length', $length);
             }

src/Symfony/Component/HttpKernel/HttpCache/HttpCache.php

@@ -581,7 +581,7 @@ protected function store(Request $request, Response $response)
     private function restoreResponseBody(Request $request, Response $response)
     {
         if ('HEAD' === $request->getMethod() || 304 === $response->getStatusCode()) {
-            $response->setContent('');
+            $response->setContent(null);
             $response->headers->remove('X-Body-Eval');
             $response->headers->remove('X-Body-File');