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'm not sure we win a lot by having the extra helper here. From the name it's not really clear what the method does (*invalid* could mean many different things in this context). It might be better to just specify the headers explicitly.