upnp: fix sort by upnp:rating
upnp: ignore "Unknown" as a single genre as it is added by Platinum
CDateTime: extend SetFromW3CDate() and add GetAsW3CDate(), GetAsW3CDateTime() and SetFromW3CDateTime()
upnp: fix serialization/deserialization of datetime properties
Merge pull request #4606 from Montellese/upnp_fixes

upnp: some fixes