Random key from Ruby Hash

I needed a Ruby Hash class that could extract a random key from the keys that have not been extracted before. Plus that the class should be re-entrant...

I found baby's class, but it was not working (at least not in Ruby 1.8.x). I made it work (slight changes), and I added the keys_not_used to make it work faster (baby's random would cycle almost forever on large lists). But I would say all credit should go to the original author, who did all the hard work.