change_subscription called only when necessary

When we are using subscription by pattern change subscription is
called every metadata update even when nothing changes. This PR
ensures that change subscription is called only when set of topics
changes.

Author: Petr Šebek

kafka/coordinator/consumer.py

@@ -140,8 +140,9 @@ def _handle_metadata_update(self, cluster):
                 if self._subscription.subscribed_pattern.match(topic):
                     topics.append(topic)
 
-            self._subscription.change_subscription(topics)
-            self._client.set_topics(self._subscription.group_subscription())
+            if set(topics) != self._subscription.subscription:
+                self._subscription.change_subscription(topics)
+                self._client.set_topics(self._subscription.group_subscription())
 
         # check if there are any changes to the metadata which should trigger
         # a rebalance