data-type is always 'text/html'

It seems that the textarea `data-type` attribute is always `text/html`. Is this intended?

This is because the `Content-Type` header is set to `text/html` by the middleware before the textarea element is rendered. Even the example on the Readme has has a `data-type` of `text/html` even though it is passing JSON data inside.

The current version of https://github.com/cmlenz/jquery-iframe-transport/ seems to require the `data-type` to be set to `application/json` in order for the JSON data to be returned in the `done` callback of a jQuery `$.ajax(..)` request. Otherwise I just get raw HTML that I then have to extract the JSON data from. Am I missing something?