Superfluous "charset" parameter in JSON content-type

[Philonous]

7793b52 adds "charset=utf-8" to the content type of json responses, but the parameter isn't defined in the media-type registration. This causes breakage with hspec-wai (hspec/hspec-wai#46)

Related to: #559

[phadej]

I'm sorry, we have ping-ponging this back and forth, and there's always some broken client.
Currently servant-server behaves well, as if asked for just application/json it will response with JSON.

The JSON RFC says on page 10

Note: No "charset" parameter is defined for this registration.
Adding one really has no effect on compliant recipients.

So: hspec-wai should relax its mime-type matching, e.g. by using mainType from http-media.