Fixes default interval for get_many. Closes #1563

celery/backends/base.py

@@ -367,6 +367,7 @@ class KeyValueStoreBackend(BaseBackend):
                         if value is not None)
 
     def get_many(self, task_ids, timeout=None, interval=0.5):
+        interval = 0.5 if interval is None else interval
         ids = set(task_ids)
         cached_ids = set()
         for task_id in ids: