Interoperating ActiveResource and ActionController::Responder

I've just created a pull request to finally make ActiveResource correctly handle validation errors returned from ActionController::Responder's default JSON handling code. The code also handles the format produced by pre-3.2 Responders *and* the format previously expected by ActiveResource, so it shouldn't break in any application that was working before.

https://github.com/rails/rails/pull/4881

Would appreciate any feedback - it would be nice for the framework to actually work with its own API endpoints...

Thanks,

--Matt Jones