views: add a function to return the Content-Type of the request
views: add a function to return the Content-Type of the request

This is just a small helper function that parses the Content-Type header
and strips off the ";charset" part to just return the MIME type.  It
will return `None` if the Content-Type header is not in the request.

davidfischer/rpc4django,davidfischer/rpc4django,davidfischer/rpc4django