Send dataTypes in post, useful for Rails and similar frameworks.

When we have forms that use a jQuery `$.ajax dataType` other than `text/html`, when it's submitted via the iframe-transport, it always gets overwritten as just a `text/html` request.

Of course, that's to be expected since it's being submitted as a normal form submission (not actually AJAX), but we need to preserve the original intended data-type and send it to the server so that we can handle the response appropriately.