Removed [Category("NotWorking")] from PackUriHelperTests.
CompareInvalidTest is still not working because .NET PackUriHelper.ComparePackUri is doing something that uses Uri and throws an exception whereas Mono is not.
Changed Check.PartUriIsValid to throw ArgumentException.
Added argument validations to ComparePackUri an ComparePartUri.
Merge pull request #1199 from esdrubal/packurihelper

PackUriHelper