Translate Map Values when Incompatible for Update

[geoffreywiseman]

At the moment, Moo will allow you to update a map property, but if the source value and destination value aren't compatible, it'll throw an exception. Would be nice if you could ask Moo to update the value if they're compatible, but not otherwise. Of course, deciding what's compatible is very tricky, and that's why it hasn't been done yet.