Fix testapp with ruby-2.0, handle HTTP 204 No content

testapp fails to work with ruby-2.0 because of [sprockets issue](https://github.com/sstephenson/sprockets/issues/352), I've updated gems to minimal working versions.

Also rails-4 [responds with HTTP 204 No Content](https://github.com/rails/rails/issues/9862) if `respond_with` is used and object hasn't changed. I've added this case handling and related test.