MaskedColumn finalize not giving expected results

Slicing or doing other operations on a masked column doesn't propagate the other attributes:
```
>>> c = MaskedColumn(data=[1,2], name='a', description='a')
>>> print c[1:2].description
None

>>> c = Column(data=[1,2], name='a', description='a')
>>> print c[1:2].description
a
```