http.go
func (resp *Response) WriteGzipLevel(w *bufio.Writer, level int) error {
	if err := resp.gzipBody(level); err != nil {
		return err
	}
	return resp.Write(w)
}
func (resp *Response) WriteGzipLevel(w *bufio.Writer, level int) error {
	if err := resp.gzipBody(level); err != nil {
		return err
	}
	return resp.Write(w)
}
// WriteGzipLevel writes response with gzipped body to w.
//
// Level is the desired compression level:
//
//     * CompressNoCompression
//     * CompressBestSpeed
//     * CompressBestCompression
//     * CompressDefaultCompression
//     * CompressHuffmanOnly
//
// The method gzips response body and sets 'Content-Encoding: gzip'
// header before writing response to w.
//
// WriteGzipLevel doesn't flush response to w for performance reasons.