# File lib/action_controller/base.rb, line 1129
      def fresh_when(options)
        options.assert_valid_keys(:etag, :last_modified)

        response.etag          = options[:etag]          if options[:etag]
        response.last_modified = options[:last_modified] if options[:last_modified]

        if request.fresh?(response)
          head :not_modified
        end
      end