fix tests that involve dates fail when OS culture has 24 hours format

[mnieto]

PR Summary

Inside the vsteam-lib.Test project, tests that have Asserts based on date format (i.e. actual.CreatedOn.ToString("M/d/yyyy h:mm:ss tt").) fails when ran in a Windows machine configured with 24h format, for example Spanish. See: https://stackoverflow.com/q/38976486/777551

Example of error:

The solution is to use an invariant culture so tests will be independent of the local configuration

PR Checklist

• [x] Write Help
• [x] Write Unit Test
• [x] Update CHANGELOG.md

[mnieto]

@SebastianSchuetze, can you please review last changes and merge?