code: function isFresh(req, res) {
  	if ((res.statusCode >= 200 && res.statusCode < 300) || 304 === res.statusCode) {
  		return fresh(req.headers, {
  			"etag": res.getHeader("ETag"),
  			"last-modified": res.getHeader("Last-Modified")
  		});
  	}
  	return false;
  }
docs: Check the data freshness.
  
  @param {*} req
  @param {*} res
  
  @returns {Boolean}