Fix IS_STRONG used with validate_and_update
Fix IS_STRONG used with validate_and_update

Fix no error return with IS_STRONG validator because it returns a gluon.html.XML instead of str, so when pushed to response.errors that is a Row, it isn't display because Row object filters his output and doesn't support XML objects.
stephenrauch/pydal,web2py/pydal,michele-comitini/pydal,manuelep/pydal,kmcheung12/pydal,niphlod/pydal,willimoa/pydal