Most of the time it doesn't matter, but for applications like anti-phishing, you really need the canonical version of an IP address so that it can be looked up in a list. KURL does no IP address canonicalization.
I believe we match Firefox here.
I'm going to make a giant compatibility matrix now that we have a bunch of LayoutTests for this kind of behavior.
KURL is gone. (Also, we do canonicalize these days.)