Fix option deprecation. (#93)

Author: tjprescott

knack/arguments.py

@@ -192,7 +192,7 @@ def __call__(self, parser, namespace, values, option_string=None):
                     try:
                         super(DeprecatedOptionAction, self).__call__(parser, namespace, values, option_string)
                     except NotImplementedError:
-                        setattr(namespace, dest, values)
+                        setattr(namespace, self.dest, values)
 
             return DeprecatedOptionAction