Added support for multiple default values

There is a bug in Term::UI, that one can not supply multiple default values even if "multi => 1" is set. Our software needs that, so I fixed it. I'm not sure, if my changes work with the history feature, because I know to less about that.
It would be very nice, if you could review my change and perhaps add it to official version. I'm happy to apply additional fixes, if my change is buggy.