Unsubscribe not removing hierarchy

[jbutz]

I noticed that the following example did not function as stated. I looked into the issue and found that only the topic with an exact match was removed. Then I looked into why the unit tests were passing and noticed the publish was occurring on the wrong topic to test that the hierarchy had been removed.

PubSub.subscribe('a', myFunc1);
PubSub.subscribe('a.b', myFunc2);
PubSub.subscribe('a.b.c', myFunc3);

PubSub.unsubscribe('a.b');
// no further notications for 'a.b' and 'a.b.c' topics
// notifications for 'a' will still get published

[mroderick]

Thank you for your investigation and your PR! 👍