File URI's without a host are not UNC paths on Windows.

NUnit breaks when these URI's are treated as UNC paths, so I have no way of
running the tests on Windows without this change to see if it broke anything.
Not all URI tests pass, but the ones that fail seem unrelated to my change.

On Unix, the tests all work, but of course on Unix this is a no-op.

This commit licensed as MIT/X11.
Merge pull request #1360 from madewokherd/winuncpath

File URI's without a host are not UNC paths on Windows.