code: func (r Reader) Render(w http.ResponseWriter) (err error) {
  	r.WriteContentType(w)
  	r.Headers["Content-Length"] = strconv.FormatInt(r.ContentLength, 10)
  	r.writeHeaders(w, r.Headers)
  	_, err = io.Copy(w, r.Reader)
  	return
  }
docs: // Render (Reader) writes data with custom ContentType and headers.