Test that consumer.subscription() returns a copy

test/test_consumer.py

@@ -24,6 +24,14 @@ class TestKafkaConsumer(unittest.TestCase):
         with self.assertRaises(KafkaConfigurationError):
             KafkaConsumer(bootstrap_servers='localhost:9092', fetch_max_wait_ms=41000, request_timeout_ms=40000)
 
+    def test_subscription_copy(self):
+        consumer = KafkaConsumer('foo', api_version=(0, 10))
+        sub = consumer.subscription()
+        assert sub is not consumer.subscription()
+        assert sub == set(['foo'])
+        sub.add('fizz')
+        assert consumer.subscription() == set(['foo'])
+
 
 class TestMultiProcessConsumer(unittest.TestCase):
     @unittest.skipIf(sys.platform.startswith('win'), 'test mocking fails on windows')