Fix bug 998 - in XmlResolver.ResolveUri(), make some policy change on "how to determi...

Fix bug 998 - in XmlResolver.ResolveUri(), make some policy change on "how to determine a string as an absolute URI.

It's a bit difficult decision, as the "relative URI" could be any path string
that contain colons as a *valid* path (especially on *nix land).
We historically did allow such string that contain "http:" etc. as to be
treated as absolute (which historically worked long time).
But this test case uncovered possible other cases.
Hopefully this does not bring issues that is pretty much anticipated. Such
issues could also happen in older code anyways.