fixed default validators ofr string and boolean

gluon/dal.py

@@ -75,7 +75,7 @@ def _default_validators(db, field):
     if field.unique:
         requires.append(validators.IS_NOT_IN_DB(db, field))
 
-    if field.notnull or field.unique:
+    if field.notnull or field.unique and not field_type in ['string','boolean']:
         requires.insert(0,validators.IS_NOT_EMPTY())
     elif not field.notnull and requires:
         requires[0] = validators.IS_EMPTY_OR(requires[0])