    # 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