Workaround for URLConnection caching auth.

With drop of HC3 we also don't use the Restlet Client
powered by HC3 but a fallback to one using java.net.URL.
But, that one suffers from the "auth caching" problem,
so added a workaround that clears URLConnection auth
caches for now.

Fixme: as the workaround uses sun... package,
will emit warnings, but this is the only way to
make URLConnection use different (or none after using
some) auth info in subsequent invocations against same
host/port combo.