table: remove_column changes structure in some way, so that other functions become unavailable

After calling `table.remove_column` the way the internal data of talbe is hold changes in some way. In some cases I now get a np.ndarray where I really should get table.Column object. This mean that some of the table functionality (e.g. setting a fill_value) suddenly becomes unavailable.

```
In [1]: from astropy.table import Table

In [2]: t = Table([(1, 2), (3, 4)], names=('a', 'b'), masked=True)
In [3]: t.remove_column('b')

In [4]: t['a'].fill_value = -99
ERROR: AttributeError: 'numpy.ndarray' object has no attribute 'fill_value' [astropy.table.table]
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-62c0945903ab> in <module>()
----> 1 t['a'].fill_value = -99

/data/guenther/anaconda/lib/python2.7/site-packages/astropy-0.3.dev4389-py2.7-linux-x86_64.egg/astropy/table/table.pyc in fill_value(self, val)
    666 
    667         if self.parent_table:
--> 668             self.parent_table._data[self._name].fill_value = val
    669 
    670         # Yet another ma bug workaround: If the value of fill_value for a string array is

AttributeError: 'numpy.ndarray' object has no attribute 'fill_value'
```