Owned partitions in the subscription must be sorted

While investigating https://issues.apache.org/jira/browse/KAFKA-12896, I have noticed that the leader was always sending the same subscribed partitions but not always in the same order. The group coordinator compares the provided subscription with the store subscription based on their bytes representation. So if the subscribed partitions are not in the same order, the group coordinator would consider that they are different and rebalance the group.