This was in the wrong place - JSON formatters have their own options
type already.
Moved the option to MvcJsonOptions and updated the naming + defaults to
reflect our plan.
Also did a bunch of general cleanup on these tests, which were a bit
sloppy.
* Add an UnsupportedContentType to the ModelState dictionary when no formatter can read the body.
* Add a filter to the pipeline that searches for that specific exception and transforms the response into 415.