Fix missing profiling header on ajax requests

On ajax requests, the xhprof profiler stops in a previous listener (not in Response listener).
Because of that, the `$this->collector->stopProfiling` call returned wrong run id (link) in the `onCoreResponse` event.

DataCollector/XhprofCollector.php

@@ -98,7 +98,7 @@ private function getFlags()
     public function stopProfiling($serverName, $uri)
     {
         if (!$this->collecting) {
-            return false;
+            return $this->data['xhprof'] ? $this->data['xhprof'] : false;
         }
 
         $this->collecting = false;