Fixed status code errors by calling _process_options

Needed to use the `_process_options` method explicitly because the overriding of `render_to_body` in the `ControllerAdditions` module removes it from the call chain.

Should resolve https://github.com/apotonick/roar-rails/issues/71
It's pretty hard to step through, but here's the gist of what's going on:

The `render_to_body` method we define in `ControllerAdditions` overrides the behavior in `ActionController::Renderer`.  The original method calls `_handle_render_options` which searches for a key like `:json` and if it finds it calls `_process_options` and  `_render_option_json`.  This is the point where Active Model Serializers would hook into the process (by overriding the method `_render_option_json`) and instantiating a serializer which responds to `to_json`.

I'd say a better implementation of this would be to copy what AMS does (https://github.com/rails-api/active_model_serializers/blob/master/lib/action_controller/serialization.rb#L43), provide custom `_render_option_json` (and maybe `_render_option_xml`) implementations that deal with converting from `Roar::Rails::Responder::Response` into a response string.  This gets out of the way and lets Rails deal with the rest of the response behavior.  What do you think of that @apotonick?