Add fallback to None if no __values__ method

lib/nntp/tiny/db.py

@@ -67,7 +67,7 @@ class Database():
         sql += ", ".join(['?' for c in table.columns if c != table.key])
         sql += ')'
 
-        fn = getattr(obj, '__values__')
+        fn = getattr(obj, '__values__', None)
 
         if fn is not None:
             values = fn()