Feature/content type query param override

Adds a `URL_CONTENTTYPE_OVERRIDE` as suggested in [Override content type?](https://github.com/tomchristie/django-rest-framework/issues/1676)

This feature will allow users to override the content type via a query param, like: `/?content_type=application/x-www-form-urlencoded`. This comes in handy when you're unable to control the content type being sent over. For example, I'm using it to override 'text/plain' content type header sent by [jQuery-ajaxTransport-XDomainRequest](https://github.com/MoonScript/jQuery-ajaxTransport-XDomainRequest/).