Unexpected valid directory assertion check

It says [here](https://github.com/fluttercommunity/flutter_downloader/blob/master/lib/src/downloader.dart#L89) that if ```saveInPublicStorage``` is set to ```true```, ```saveDir``` will be ignored, meanwhile there's still an [assertion](https://github.com/fluttercommunity/flutter_downloader/blob/master/lib/src/downloader.dart#L105) at the start of the function that throws an error if ```saveDir``` is invalid.