Shared Subscription does not match topic using topic_matches_sub()

Using release 1.5.0

I would expect the behavior of this call to return True.
`topic_matches_sub("$share/group/+/+/c", "a/b/c")`
The `MQTTMessage.topic` received from the `on_message` callback will be the second part (a/b/c) and is a topic that should fall within the shared subscription topicFilter.

I believe this is causing issues with Client.message_callback_add(). If the subscription is a shared subscription (E.g. `Client.message_callback_add("$share/group/+/+/c", callback)` the callback is never executed.