Can't rename table columns after sorting

After sorting a table on one column, it is no longer possible to rename column names, apparently.

```
In [51]: %paste
>>> from astropy.table import Table
>>> a = [1, 4, 5]
>>> b = [2.0, 5.0, 8.2]
>>> c = ['x', 'y', 'z']
>>> t = Table([a, b, c], names=('a', 'b', 'c'), meta={'name': 'first table'})

## -- End pasted text --

In [52]: t.rename_column('c','d')

In [53]: t.colnames
Out[53]: ['a', 'b', 'd']

In [54]: t.sort('a')

In [55]: t.colnames
Out[55]: ['a', 'b', 'd']

In [56]: t.rename_column('b','e')

In [57]: t.colnames
Out[57]: ['a', 'b', 'd']
```