Decide on the behaviour of default/empty URL fields

[smola]

I've changed to match WHATWG and W3C specs here, but that's an important deviation from java.net.URI and java.net.URL (including Android implementation).

java.net.URI, since it performs no normalization operation at all, accepts both empty and null userInfo. In our case, userInfo is never null. The same happens with authority and other fields.

We probably want this to stay this way, but some warnings to javadocs of the corresponding methods would be nice.