Fix default validator when unique=True
Fix default validator when unique=True

Append the IS_NOT_IN_DB validator so it is applied to the final value after any previous validators may have altered the original value.
stephenrauch/pydal,manuelep/pydal,niphlod/pydal,web2py/pydal,michele-comitini/pydal,kmcheung12/pydal,willimoa/pydal