CurrentFile.SaveAs() doesn't work for untitled files

[rjmholt]

If an untitled file is opened and the language mode changed to PowerShell, trying to SaveAs() will silently do nothing because the first thing we do is check for the file path. Untitled URIs fail this test, so just do nothing.

We should instead recognise untitled URIs and save them relative to the workspace root (or if an absolute path is given, just save it there).

Thanks to @ brwilkinson to pointing this issue out.