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.)
This was accidentally left over from when we were trying out `respond_with`, based on https://github.com/rails-api/rails-api/issues/93. Rebased to remove it.