code: func (h *HTTPBin) Cache(w http.ResponseWriter, r *http.Request) {
  	if r.Header.Get("If-Modified-Since") != "" || r.Header.Get("If-None-Match") != "" {
  		w.WriteHeader(http.StatusNotModified)
  		return
  	}
  
  	lastModified := time.Now().Format(time.RFC1123)
  	w.Header().Add("Last-Modified", lastModified)
  	w.Header().Add("ETag", sha1hash(lastModified))
  	h.Get(w, r)
  }
docs: // Cache returns a 304 if an If-Modified-Since or an If-None-Match header is
  // present, otherwise returns the same response as Get.