Specify MimeType to always use for ajax templates

[MasterOdin]

For twig.js to be used in the browser, it requires the user to know that they either must name their templates as an ending that browsers recognized (e.g. txt or html) or to manually register a mime-type to the .twig extension. Otherwise, when fetching a template, a browser will attempt to parse it using text/xml which can cause issues depending on strictness (e.g. #741).

This PR changes it so that twig.js always specifies that templates received via XMLHttpRequest are text/plain and to be treated as such.

[RobLoach]

This makes sense to me. I can't think of issuing any other mime type through this. Thoughts Poly?

[MasterOdin]

Any chance on seeing this get into a release in the near term? As is, without manually setting the content type through apache / nginx, twig.js is broken for the latest Firefox users for us.