[bz 6440446] Bug with cancel upload

[JetFault]

When canceling an upload right as it finishes, cancelUpload throws a TypeError because this.get('xhr') is null.

[JetFault]

also fixed typo where uploadcancel event wasn't being fired in commit 9db6351 :(

[ericf]

Instead of a try/catch, could you just check whether or not the result of calling get('xhr') is truthy before calling abort()?

var xhr = this.get('xhr');
if (xhr) {
    xhr.abort();
}

[ericf]

@JetFault Thanks for working on this. I added a comment about the try/catch above. Could you also add a test and HISTORY.md entry for this?

[JetFault]

@ericf added tests, changed to if/else

[JetFault]

@ericf Any reason why this isn't getting merged in?

[clarle]

@ericf This looks good to me, I'll handle the merge conflicts for getting this in if you sign this off.

[clarle]

@JetFault I re-tested this on IE, Chrome, and Firefox and all looks good. Fixed up some of your HISTORY.md entries but otherwise merged in as-is.

Thanks again for the contribution, we really appreciate it!