[HttpFoundation] replace any preexisting Content-Type headers

src/Symfony/Component/HttpFoundation/Response.php

@@ -344,8 +344,9 @@ class Response
 
         // headers
         foreach ($this->headers->allPreserveCase() as $name => $values) {
+            $replace = 0 === strcasecmp($name, 'Content-Type');
             foreach ($values as $value) {
-                header($name.': '.$value, false, $this->statusCode);
+                header($name.': '.$value, $replace, $this->statusCode);
             }
         }