Added application/rdf for file extension rng in mimedetect.

**application/xml** or **text/xml** according to [Wikipedia](http://en.wikipedia.org/wiki/RELAX_NG). IANNA has nothing to say on the [subject](http://www.iana.org/assignments/media-types/media-types.xml) (Search for Relax NG and only the compact syntax has it's own mimetype).