fix shutdown bug

Worker exits from the consume loop when shutdown is requested.
When the loop is exited the channel is closed
amqp library calls drain_events method.
There may be received messages but since the channel is closed
we cannot process them.

kuyruk/worker.py

@@ -158,6 +158,9 @@ def _cancel_queues(self, ch):
 
     def _process_message(self, message):
         """Processes the message received from the queue."""
+        if self.shutdown_pending.is_set():
+            return
+
         try:
             description = json.loads(message.body)
         except Exception: