Column with no name cannot be printed

@taldcroft - Currently, printing a `Column` with its name not set gives a `TypeError`:
```
from astropy.table import Column
print(Column([1.,2.]))
ERROR: TypeError: object of type 'NoneType' has no len() [astropy.table.pprint] 
```
This can be fixed quite easily in `pprint.py`, in `_pformat_col_iter`, by setting the heading for the column to `''` or a space or so when `name is None`. But before I do so, I thought I would first check that this would be the idea.