Error with a 415 code if the Content-Type is not application/json

- This uses a `before_filter` in ApplicationController to check
  Content-Type, leaving `respond_to` to deal with the Accept header.

(Paired with @benilovj and @rgarner on this, ish.)
I wouldn't expect them to return the same, rather:

* `406` if `Accept` header is inappropriate
* `415` if `Content-Type` is anappropriate