code: function getCompressedStream(req, res) {
    var encoding = req.headers['accept-encoding'] || '';
    var stream;
    var contentEncodingHeader = res.getHeader('Content-Encoding');
    if ((contentEncodingHeader === 'gzip') || /\bgzip\b/.test(encoding)) {
      if (!contentEncodingHeader) {
        res.setHeader('Content-Encoding', 'gzip');
      }
      stream = zlib.createGzip();
      stream.pipe(res);
    } else if ((contentEncodingHeader === 'deflate') ||
        /\bdeflate\b/.test(encoding)) {
      if (!contentEncodingHeader) {
        res.setHeader('Content-Encoding', 'deflate');
      }
      stream = zlib.createDeflate();
      stream.pipe(res);
    }
    return stream;
  }
docs: Return a writable response stream, using compression when possible.