Fix #1370 - Always use the provided formatter in JsonResult

The change here is to always use the provided formatter, instead of using
it as a fallback. This is much less surprising for users.

There are some other subtle changes here and cleanup of the tests, as well
as documentation additions.

The primary change is that we still want to run 'select' on a formatter
even if it's the only one. This allows us to choose a content type based
on the accept header.

In the case of a user-provided formatter, we'll try to honor the best
possible combination of Accept and specified ContentTypes (specified
ContentTypes win if there's a conflict). If nothing works, we'll still run
the user-provided formatter and let it decide what to do.

In the case of the default (formatters from options) we do conneg, and if
there's a conflict, fall back to a global (from services)
JsonOutputFormatter - we let it decide what to do.

This should leave us with a defined and tested behavior for all cases.
Why not make the property a getter only? Saves the trouble of having to do null checks.