masked table assignment with [row][column] indexing

It seems that the masked table object doesn't work properly when being assigned in t[row][col] order. (See example below, which results in proper assignment if table is not masked or if I assign in [col][row] order.) I'm using v0.3.2 of astropy.

```python
In [13]: t = Table(np.array([(1, 1.0), (2, 2.0)], dtype=[('a',int), ('b', float)]),masked=True)

In [14]: t
Out[14]: 
<Table rows=2 names=('a','b')>
masked_array(data = [(1, 1.0) (2, 2.0)],
             mask = [(False, False) (False, False)],
       fill_value = (999999, 1e+20),
            dtype = [('a', '<i8'), ('b', '<f8')])

In [16]: t[1]['a']
Out[16]: 2

In [17]: t[1]['a']=3

In [18]: t[1]['a']
Out[18]: 2
```python