Impossible to pretty-print response bodies by using `pretty=true` media type parameter

[p-himik]

https://github.com/dakrone/cheshire/pull/100 has made a backwards incompatible change that requires the :pretty option for json/encode to be either a boolean, a map, or an instance of PrettyPrinter. But yada.media-type/string->media-type does not convert any properties from strings except for the quality. Meaning, the "pretty" parameter will be a string "true". In this case, cheshire just ignores the :pretty option altogether.