parse req.type by media-typer

not sure if `this.request.charset` is really useful. but we can use [media-typer](https://github.com/expressjs/media-typer) to do this. 
Really, whatever is doing the body parsing should be decoding the charset (if applicable). Not sure how useful `req.charset` _really_ is, but I guess I can see people using it for some reason.

Plus since this `req.charset` is a getter, what it should to is just on first call, replace itself with the parsed value. That would probably be good enough for caching.