fixed json field representation

gluon/dal.py

@@ -2037,7 +2037,7 @@ class BaseAdapter(ConnectionPool):
             else:
                 obj = str(obj)
         elif fieldtype == 'json':
-            if not self.native_json:
+            # always pass a string JSON string
             if have_serializers:
                 obj = serializers.json(obj)
             elif simplejson: