Fix $.fn.val issue on IE9

This is an issue with the jquery.adapter on IE9 when Placeholders#enable is used, like after an ajax submission.

Basically, when a form is refreshed because of an ajax response, the placeholders are gone in IE9. So we need to call Placeholders#enable. When doing that, $.fn.val() returns an empty string value for the fields that were blank before the submission, even if we change the contents later.

This commit is a workaround that fixes the issue.