fix #451 Change test url in jsonp test

We used to target twitter for testing real jsonp requests. Lately twitter API became rate limited and thus failing from time to time.
This changes to jsfiddle echo service.
(cherry picked from commit 85717e0970a8862a159e9f3492204220c05c0bd9)
fix #570 Wrong content type in IE 8 with asyncFormSubmit.

In IE 8, setting the enctype attribute of a form with the
following line does not work:
 form.enctype = value;

The following line works better:
 form.setAttribute("enctype", value);
(cherry picked from commit 5e8ebc19c8d51b24c4ec6be4168d8d7febc2e14d)
1.3.7D