Avoid confusing semantics when comparing delete rules.

mongoengine/base.py

@@ -191,9 +191,10 @@ class DocumentMetaclass(type):
         new_class = super_new(cls, name, bases, attrs)
         for field in new_class._fields.values():
             field.owner_document = new_class
-            if hasattr(field, 'delete_rule') and field.delete_rule > DO_NOTHING:
+            delete_rule = getattr(field, 'delete_rule', DO_NOTHING)
+            if delete_rule != DO_NOTHING:
                 field.document_type.register_delete_rule(new_class, field.name,
-                        field.delete_rule)
+                        delete_rule)
 
         module = attrs.get('__module__')