Registering mimetype documentation update

[alvincrespo]

The docs state to register the mime type "application/vnd.api+json" with the Mime::Type module for params to be parsed correctly, but unfortunately this did not work for me.

I had to unregister the json mime type and then re-register it:

Mime::Type.unregister(:json)
Mime::Type.register(JSONAPI::MIMETYPE, :json)

I'm using Rails 5.0.0.1. Has anyone else encountered this issue? Should the docs reflect this solution?