resume method
Resume file downloading, if file exists downloading will continue from file size
will return false
if downloading is in progress, finished or cancelled
Implementation
Future<bool> resume() async {
if (_doneOrCancelled || _downloading) return false;
_addEvent(TaskEvent(
state: TaskState.downloading,
bytesReceived: _bytesReceived,
totalBytes: _totalBytes));
_download().then((value) => _subscription = value);
// _subscription = await _download();
return true;
}