abort() should not fire error event #25

I just noticed that in one of my tests, calling xhr.abort() fires the error event, but this does not happen in Chrome or in Safari (only fires the abort event).

Looking at the tests, it's clear that this is the intended behavior.

The way I'm interpreting the spec, it doesn't appear that abort() should fire the onerror event, but there is definitely some ambiguity I can't quite work though.

https://www.w3.org/TR/XMLHttpRequest/#the-abort()-method

Finally I noticed that this bug appears to have been fixed in sinon's FakeXMLHttpRequest implementation here: sinonjs/sinon#861.

If you agree that this behavior should be changed, let me know and I can create a PR.