code: func (req *Request) BodyWriteTo(w io.Writer) error {
  	if req.bodyStream != nil {
  		_, err := copyZeroAlloc(w, req.bodyStream)
  		req.closeBodyStream()
  		return err
  	}
  	if req.onlyMultipartForm() {
  		return WriteMultipartForm(w, req.multipartForm, req.multipartFormBoundary)
  	}
  	_, err := w.Write(req.bodyBytes())
  	return err
  }
docs: // BodyWriteTo writes request body to w.