Check for invalid filenames when using username as a filename in uwp

[csteeg]

… check/generate a valid filename

Xamarin.Auth Pull Request

Fixes #325 .

Checklist

• [X] I have included examples or tests (standard sample now works with all usernames)
• [ ] I have updated the change log
• [ ] I am listed in the CONTRIBUTORS file
• [X] I have cleaned up the commit history (use rebase and squash)

Changes proposed in this pull request:

• strip invalid characters before creating a file based on username

[dnfclas]

All CLA requirements met.

[moljac]

Approved/Merged Simple changes whcih can be easily changed during tests. Will be included in 1.6.0.5 or 1.7.0.0 depending on other fixes and PRs