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