code: func (w *writer) WriteChunked(r io.Reader) error {
  	if w.phase != body {
  		return &phaseError{body, w.phase}
  	}
  	cw := httputil.NewChunkedWriter(w)
  	if _, err := io.Copy(cw, r); err != nil {
  		return nil
  	}
  	w.phase = requestline
  	return cw.Close()
  }
docs: // WriteChunked writes the contents of r in chunked format to the wire.