Delete messages from batch after successful commit API return.

Addresses: https://github.com/GoogleCloudPlatform/gcloud-python/pull/742/files#r26701080

Author: tseaver

gcloud/pubsub/test_topic.py

@@ -149,6 +149,7 @@ def test_publish_multiple(self):
             batch.publish(PAYLOAD1)
             batch.publish(PAYLOAD2, attr1='value1', attr2='value2')
         self.assertEqual(list(batch), [MSGID1, MSGID2])
+        self.assertEqual(list(batch.messages), [])
         self.assertEqual(len(conn._requested), 1)
         req = conn._requested[0]
         self.assertEqual(req['method'], 'POST')

gcloud/pubsub/topic.py

@@ -145,5 +145,6 @@ def commit(self):
         conn = self.topic.connection
         response = conn.api_request(method='POST',
                                     path='%s:publish' % self.topic.path,
-                                    data={'messages': self.messages})
+                                    data={'messages': self.messages[:]})
         self.message_ids.extend(response['messageIds'])
+        del self.messages[:]