fetch method Null safety
We should implement this method to make real http requests.
options: The request options
requestStream The request stream, It will not be null
only when http method is one of "POST","PUT","PATCH"
and the request body is not empty.
We should give priority to using requestStream(not options.data) as request data.
because supporting stream ensures the
cancelFuture: When cancelled the request,
cancelFuture will be resolved!
you can listen cancel event by it, for example:
cancelFuture: will be null when the request is not set CancelToken.
Future<ResponseBody> fetch( RequestOptions options, Stream<Uint8List>? requestStream, Future? cancelFuture, );