[XHR2] xhr.upload and abort and error events

Hi all,

currently per the draft 'abort' and 'error' events are always
dispatched to .upload, even if there isn't anything to upload.
'loadstart', 'progress' and 'load' are dispatched only if
request entity body is not empty.
I think the same restriction should apply to 'abort' and 'error' too.

One way to solve this and the upload complete flag handling problem is
to set the flag to true in send() if request body is empty.

-Olli