code: func RangeHeader(r *http.Request) (headerutil.Range, error) {
  	header := r.Header.Get("Range")
  	if header == "" {
  		return headerutil.Range{}, nil
  	}
  
  	rangeHeader, err := headerutil.ParseRange(header)
  	if err != nil {
  		return headerutil.Range{}, err
  	}
  	return *rangeHeader, nil
  }
docs: // RangeHeader parses the Range header and returns an headerutil.Range.