Improper use of fm.join when joining parts of a url in `CDataLoaderMNIST`

We wrongly used fm.join when creating the url from where the MNIST data should be downloaded in CDataLoaderMNIST as part of the recent update !624. This is problematic on windows, where the folder separator is "" instead of "/".

While fm.join is perfect for paths, URLs should be joined only using explicit "/". So we replace use of fm.join in _get_data with a "+".