http.go
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
}
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
}
// BodyWriteTo writes request body to w.