Pop _unique_id when checking for duplicates

Drivers should not be returning messages with any driver-specific
wire format fields included. Right now, the rabbit and qpid drivers are
leaking the _unique_id field.

Avoid this simply by popping _unique_id when we're checking for dups.

Change-Id: Icbfb02ee66aebe5ef97a9a2502d8a0745e07bd8b

oslo/messaging/_drivers/amqp.py

@@ -323,7 +323,7 @@ class _MsgIdCache(object):
            before ack is returned. This method prevents doing it.
         """
         if UNIQUE_ID in message_data:
-            msg_id = message_data[UNIQUE_ID]
+            msg_id = message_data.pop(UNIQUE_ID)
             if msg_id not in self.prev_msgids:
                 self.prev_msgids.append(msg_id)
             else:

tests/test_rabbit.py

@@ -52,7 +52,4 @@ class TestRabbitDriver(test_utils.BaseTestCase):
         received = listener.poll()
         self.assertTrue(received is not None)
         self.assertEquals(received.ctxt, {})
-
-        # FIXME(markmc): this should be done by the driver
-        received.message.pop('_unique_id')
         self.assertEquals(received.message, {'foo': 'bar'})