Raise exception if given a bad topic name (#824)

kafka/consumer/subscription_state.py

@@ -128,15 +128,22 @@ class SubscriptionState(object):
 
         Raises:
             IllegalStateErrror: if assign_from_user has been used already
+            TypeError: if a non-str topic is given
         """
         if self._user_assignment:
             raise IllegalStateError(self._SUBSCRIPTION_EXCEPTION_MESSAGE)
 
+        if isinstance(topics, str):
+            topics = [topics]
+
         if self.subscription == set(topics):
             log.warning("subscription unchanged by change_subscription(%s)",
                         topics)
             return
 
+        if any(not isinstance(t, str) for t in topics):
+            raise TypeError('All topics must be strings')
+
         log.info('Updating subscribed topics to: %s', topics)
         self.subscription = set(topics)
         self._group_subscription.update(topics)