serialize method
Implementation
Future<MultiPartBodySerializationResult> serialize() async {
final boundary =
this.boundary ?? getRandomBoundaryString(random ?? Random());
final partsList = await parts.toList();
return MultiPartBodySerializationResult(
contentLength: contentLength(partsList, boundary),
bodyBytesStream: bodyBytesStream(partsList, boundary),
boundary: boundary,
);
}