Merge pull request #43 from jcchavezs/fix_error_tag_status_code

Fix the error tag for when using the status code.

src/ZipkinBundle/Middleware.php

@@ -122,7 +122,7 @@ public function onKernelTerminate(PostResponseEvent $event)
 
         $statusCode = $event->getResponse()->getStatusCode();
         if ($statusCode > 399) {
-            $span->tag(Tags\ERROR, $statusCode);
+            $span->tag(Tags\ERROR, (string) $statusCode);
         }
 
         $span->tag(Tags\HTTP_STATUS_CODE, (string) $statusCode);