handle_conditional_get!

# File actionpack/lib/action_dispatch/http/cache.rb, line 88
        def handle_conditional_get!
          if etag? || last_modified? || !@cache_control.empty?
            set_conditional_cache_control!
          elsif nonempty_ok_response?
            self.etag = body

            if request && request.etag_matches?(etag)
              self.status = 304
              self.body = []
            end

            set_conditional_cache_control!
          else
            headers["Cache-Control"] = "no-cache"
          end
        end