function respondFromCache(req, res, cacheEntry) {
  var status = cacheEntry[0]
    , headers = utils.merge({}, cacheEntry[1])
    , content = cacheEntry.slice(2);

  headers.age = (new Date - new Date(headers.date)) / 1000 || 0;

  switch (req.method) {
    case 'HEAD':
      res.writeHead(status, headers);
      res.end();
      break;
    case 'GET':
      if (utils.conditionalGET(req) && fresh(req.headers, headers)) {
        headers['content-length'] = 0;
        res.writeHead(304, headers);
        res.end();
      } else {
        res.writeHead(status, headers);

        function write() {
          while (content.length) {
            if (false === res.write(content.shift())) {
              res.once('drain', write);
              return;
            }
          }
          res.end();
        }

        write();
      }
      break;
    default:
      // This should never happen.
      res.writeHead(500, '');
      res.end();
  }
}
Respond with the provided cached value.
TODO: Assume 200 code, that's iffy.

@param {Object} req
@param {Object} res
@param {Object} cacheEntry
@return {String}
@api private