Fixes exceptions deserialzation with amqp result backend join_native (Issue #2409)

celery/backends/amqp.py

@@ -176,7 +176,8 @@ class AMQPBackend(BaseBackend):
                 raise self.BacklogLimitExceeded(task_id)
 
             if latest:
-                payload = self._cache[task_id] = latest.payload
+                payload = self._cache[task_id] = self.meta_from_decoded(
+                    latest.payload)
                 latest.requeue()
                 return payload
             else: