Fix unobstrusive ajax  to use the right context in callbacks

The request invoker in jquery.unobstrusive-ajax.js did not specify a
context. Consequently callback functions specified using AjaxOptions would
be executed in the context of the Ajax options object which is
uninteresting. Fixing this so that the context points to the triggering
element.

Work Item: 1487

(cherry picked from commit 8a2c969ab6b41591e6a7194028b5b37a562c855a)