Form submitted without the input[type='submit'] value in authcache.js

Hello,

I've had some problems with authcache with its way to handle forms:
In order to update the token_id of a form, authcache invalidates the click on a submit button to modify the form's token_id via an AJAX call. When the AJAX call is complete, authcache submits the form by calling form.submit().

But by doing so, the form is submitted without the clicked button's name&value. That's an issue in some cases: with forms that contain several submit buttons for instance, because we need the clicked submit button's name&value to know which one was clicked.

Here is a patch that adds a hidden input containing the clicked submit button. Solution to be reviewed.