change_subscription warning log to info

When we are using subscription by pattern change subscription is
called every metadata update even when nothing changes. It logs
warning every time topics are not changed.

Author: Petr Šebek

kafka/consumer/subscription_state.py

@@ -137,8 +137,8 @@ def change_subscription(self, topics):
             topics = [topics]
 
         if self.subscription == set(topics):
-            log.warning("subscription unchanged by change_subscription(%s)",
-                        topics)
+            log.info("subscription unchanged by change_subscription(%s)",
+                     topics)
             return
 
         if any(not isinstance(t, six.string_types) for t in topics):