Allow seamless migration from .notify(done) to mocha-as-promised

I wanted to integrate Mocha-as-promised in an app that now uses the p.notify(done) mechanic.

when adding mocha-as-promised, all these tests failed instantly because done() was called multiple times. 

This way we can easily migrate from

    it 'test', (done) ->
      p = somethingPromise()
      p.notify done

to the mocha-as-promised version

    it 'test', ->
      somethingPromise()

without having a broken suite in the meantime.