fix cancelling jobs on a subqueue

Author: Florian Perucki

mrq/basetasks/utils.py

@@ -116,7 +116,7 @@ def perform_action(self, action, query, destination_queue):
             # In this case we could also loose some jobs that were queued after
             # the MongoDB update. They will be "lost" and requeued later like the other case
             # after the Redis BLPOP
-            if list(query.keys()) == ["queue"]:
+            if list(query.keys()) == ["queue"] and isinstance(query["queue"], basestring):
                 Queue(query["queue"]).empty()
 
         elif action in ("requeue", "requeue_retry"):