app_responder.rb

class AppResponder < ActionController::Responder

  def to_format
    if get? && !resource
      controller.response.status = :not_found
    elsif has_errors?
      controller.response.status = :unprocessable_entity
    elsif post?
      controller.response.status = :created
    end

    super
  end
end

application_controller.rb

require 'rabl_responder'

class ApplicationController < ActionController::Base
  self.responder = RablResponder
  respond_to :json, :xml

  ...
end