code: func (r *Response) ContentLength() int64 {
  	for _, h := range r.Headers {
  		if strings.EqualFold(h.Key, "Content-Length") {
  			length, err := strconv.ParseInt(h.Value, 10, 64)
  			if err != nil {
  				continue
  			}
  			return int64(length)
  		}
  	}
  	return -1
  }
docs: // ContentLength returns the length of the body. If the body length is not known
  // ContentLength will return -1.