application.rb

    config.middleware.use "OldAcceptBehaviour", ["text/html", "application/xml", "text/javascript", "application/atom+xml"]

trolljosh.rb

class OldAcceptBehaviour
  def initialize(app, formats)
    @app = app
    @formats = formats
  end

  def call(env)
    current_accept_header = env["HTTP_ACCEPT"]
    if !@formats.include?(current_accept_header) && !env["HTTP_X_REQUESTED_WITH"]
      env["HTTP_ACCEPT"] = "text/html"
    end
    @app.call(env)
  end
end