responseContentLength function
Gets the size (in bytes) of the response data, using the content-length
header, or 0 if unset.
Implementation
int responseContentLength(Response? response) {
  if (response == null) return 0;
  final contentLength = response.headers.value(Headers.contentLengthHeader);
  if (contentLength != null) return _fromContentLength(contentLength);
  return 0;
}