Merge pull request python-smpplib#1 from wavemm/stefanruijsenaars-patch-1

Add mandatory attributes for Generic Nack command

Author: stefanruijsenaars

smpplib/command.py

@@ -567,6 +567,8 @@ def __init__(self, command, **kwargs):
 class GenericNAck(Command):
     """General Negative Acknowledgement class"""
 
+    params = {}
+    params_order = ()
     _defs = []
 
     def __init__(self, command, **kwargs):