Table: multi-column assignment fails silently

The documentation gives as an example for a direct element assignment by giving both column and row:
t['a'][2] = 30
Naively, one might then try
t['a','c'][2] = 30, 'a'
but this doesn't do anything, yet does not give a warning.

The lack of warning is surprising, since if I do more directly what I think should happen in __setitem__,
t._data['a','c'][2] = 30, 'a'
it throws a ValueError.  This should be caught by the except, which should in turn raise a KeyError that the column does not exist. I'm not quite sure what I'm missing.

-- Marten

(for completeness, this with astropy-0.2 under ipython; the table __setitem__ revision slated for 0.3 -- #726 -- also fails silently with the above, hence I thought it was better to start a new issue)