code: function isFresh (req, res) {
    return fresh(req.headers, {
      'etag': res.getHeader('ETag'),
      'last-modified': res.getHeader('Last-Modified')
    })
  }
docs: Determine if the cached representation is fresh.
  
  @param {object} req
  @param {object} res
  @return {boolean}
  @private