code: func GetMaxAge(r *http.Request) entry.LifeChanger {
  	return func() time.Duration {
  		cacheControlHeader := r.Header.Get("Cache-Control")
  		// headerCacheDur returns the seconds
  		headerCacheDur := entry.ParseMaxAge(cacheControlHeader)
  		return time.Duration(headerCacheDur) * time.Second
  	}
  }
docs: // GetMaxAge parses the "Cache-Control" header
  // and returns a LifeChanger which can be passed
  // to the response's Reset