Use queue if provided in kwargs (#3924)

Fix for a new Queue being instantiated regardless of whether a queue is passed in or not.

pubsub/google/cloud/pubsub_v1/subscriber/policy/thread.py

@@ -61,7 +61,7 @@ def __init__(self, client, subscription, flow_control=types.FlowControl(),
         # Create a queue for keeping track of shared state.
         if queue is None:
             queue = Queue()
-        self._request_queue = Queue()
+        self._request_queue = queue
 
         # Call the superclass constructor.
         super(Policy, self).__init__(