[System] Fixed an error in the implementation of System.Net.IPAddress.IsIPv6Teredo

The property was introduced with PR #870 but it missed to convert network byte order to host byte order, so the comparison
wasn't working correctly and the corresponding unit test failed. @marek-safar erroneously "fixed" the test in 27e07c7.

This is the corrected implementation and the original test now works as intended.