code: func hasFreshness(reqDir *RequestCacheDirectives, respDir *ResponseCacheDirectives, respHeaders http.Header, respExpires time.Time, privateCache bool) bool {
  	if !privateCache && respDir.SMaxAge != -1 {
  		return true
  	}
  
  	if respDir.MaxAge != -1 {
  		return true
  	}
  
  	if !respExpires.IsZero() || respHeaders.Get("Expires") != "" {
  		return true
  	}
  
  	return false
  }
docs: // calculate if a freshness directive is present: http://tools.ietf.org/html/rfc7234#section-4.2.1