AuthenticationHandler should use WindowsPrincipal instead of ClaimsPrincipal

[kevinchalet]

Unlike IIS's authentication handler, HttpSysServer doesn't use WindowsPrincipal and directly relies on the ClaimsPrincipal base class to represent non-empty principals. This inconsistency causes weird issues when using IsInRole(...) with HttpSysServer, as ClaimsPrincipal.IsInRole(...) doesn't have the additional logic implemented by WindowsPrincipal.IsInRole(...).

This line should be changed to use WindowsPrincipal instead of ClaimsPrincipal.

/cc @Tratcher