Fix core unsubscribe by subject

[scottf]

Fixes #1232

Context

The dispatcher api allows subscribe

1. with a subject and no message handler, subscribe("foo");, in which case it will use the message handler provided when the dispatcher was created.
2. with a subject plus and a message handler, subscribe("foo", nonDefaultHandler);.

Problem

If you unsubscribe("foo"), the dispatcher currently only unsubscribes the subscription with the default handler, meaning if you had subscribed with a non default handler, that subscriptions did not get unsubscribed. (That was the behavior reported in #1232)

Desired Behavior

The dispatcher should unsubscribe for all handlers, default or otherwise, for a subject.

Considerations

You can only have one subscription for a subject with the default handler (otherwise you just get multiple copies of the same message to that handler) but you can have as many subscriptions for a subject if you provide a non default handler.

So if you do this:

subscribe("foo"); 
subscribe("foo"); 
subscribe("foo", handler1); 
subscribe("foo", handler2);

you end up with 3 subscriptions.

Solution

Unsubscribe all subscriptions for the subject regardless of the handler.

[MauriceVanVeen]

LGTM

I suppose this is a breaking change though. Just like the test, you could subscribe on the same subject using different handlers. I don't know why you'd want to do that.. but I guess worth just calling it out in the release notes and not necessarily prompt a major version bump?

[scottf]

I considered that this might be a breaking change, but I think it's a bug that no one really noticed before because typically they don't even unsubscribe, or are done with the connection anyway so it's moot.

[scottf]

There is also another issue, and that is that it's possible to make a dispatcher without a default handler and then also subscribe without supplying a handler. The message gets dropped. I'm adding throwing an exception in this case, because it's been failing silently forever, now the user will get an exception.

[MauriceVanVeen]

LGTM