Ajax: 15160 - Fix for request aborted in ajaxSend

Bug ticket: http://bugs.jquery.com/ticket/15160

The same approach to a very similar problem was used on [line 533](https://github.com/jquery/jquery/pull/1619/files#diff-c3749d3acba09ca9ec16bb56e496408bL533)

Simply: If `done` has already been invoked for the request then we don't want to call it again.