roundtrip + some refactor

I've done some refactor, getting rid of some of the lambdas (still some left, a method that returns 3 things, etc)
Also now to_media_type returns ... A media type! surprise! 
The previous to_media_type returned a hash which is no really any other thing in the system so I have named it to_representing_hash. Because it is also not a RepresnetorHash class.
Not a great name but the proposed  'as_media_type' does not cut it for me either as it returns a hash, not anything similar to a media type.