Fix edge case with StreamedResponse where headers are sent twice

Author: Nicofuma

src/Symfony/Component/HttpFoundation/StreamedResponse.php

@@ -28,6 +28,7 @@ class StreamedResponse extends Response
 {
     protected $callback;
     protected $streamed;
+    private $headersSent;
 
     /**
      * Constructor.
@@ -44,6 +45,7 @@ public function __construct($callback = null, $status = 200, $headers = array())
             $this->setCallback($callback);
         }
         $this->streamed = false;
+        $this->headersSent = false;
     }
 
     /**
@@ -75,6 +77,22 @@ public function setCallback($callback)
         $this->callback = $callback;
     }
 
+    /**
+     * {@inheritdoc}
+     *
+     * This method only sends the headers once.
+     */
+    public function sendHeaders()
+    {
+        if ($this->headersSent) {
+            return;
+        }
+
+        $this->headersSent = true;
+
+        parent::sendHeaders();
+    }
+
     /**
      * {@inheritdoc}
      *