reinforce column immutability. I could never reproduce this error

interactively, only via python setup.py test

Author: keflavich

astroquery/vizier/core.py

@@ -4,6 +4,7 @@
 import os
 import warnings
 import json
+import copy
 
 from astropy.extern import six
 import astropy.units as u
@@ -460,7 +461,7 @@ def _args_to_payload(self, *args, **kwargs):
         # process: columns
         columns = kwargs.get('columns')
         if columns is None:
-            columns = self.columns
+            columns = copy.copy(self.columns)
         else:
             columns = self.columns + columns