Incorrect behavior of deregistration function returned from $rootScope.$on when called twice

[dmytroyarmak]

Hello,

I've notice that when you call deregistration function more than one time it decrease $$listenerCount each time.
https://github.com/angular/angular.js/blob/master/src/ng/rootScope.js#L1105-L1108

After $$listenerCount becomes zero $breadcast starts ignore this event and It cause that other listeners for the same event will not be called.
https://github.com/angular/angular.js/blob/master/src/ng/rootScope.js#L1219

Reproducing

Here is plunker and steps to reproduce:
http://plnkr.co/edit/bHtWlu?p=preview

We have page with two handlers that bound to "sampleEvent", 3 buttons that broadcasts and unbinds handlers, block with displayed $$listenerCount value.

1. sampleEvent count equal 2
2. Click "Emit simple event"
3. Two alerts should appear
4. Click "Unregister first handler"
5. sampleEvent count equal 1
6. Click "Emit simple event"
7. One alert should appear
8. Click "Unregister first handler" second time

Actual result:

There is no sampleEvent count and second handler is not triggered after broadcast 'sampleEvent'

Expected result:

sampleEvent should still be 1 and second handler should be triggered after broadcast 'sampleEvent'

I'm going to create pull request with fix for this bug after few minutes.

[petebacondarwin]

Closing in favour of the pr.