http.go
func (resp *Response) BodyWriter() io.Writer {
	resp.w.r = resp
	return &resp.w
}
func (resp *Response) BodyWriter() io.Writer {
	resp.w.r = resp
	return &resp.w
}
// BodyWriter returns writer for populating response body.
//
// If used inside RequestHandler, the returned writer must not be used
// after returning from RequestHandler. Use RequestCtx.Write
// or SetBodyStreamWriter in this case.