There maybe a simpler way to handle this, rather than setting custom
form request headers*.
Have you tried setting the charset for the page containing the form to
* Which generally speaking, you can set during the beforeSubmitEvent.
However you won't be able to over-ride the default "Content-Type" used
for async form submission using Connect.initHeader, because
Connect.setForm resets the "Content-Type" header, and it gets invoked
after the beforeSubmitEvent.