Table.convert_unicode_to_bytestring doesn't work

@taldcroft [Table.convert_unicode_to_bytestring](http://astropy.readthedocs.org/en/latest/_modules/astropy/table/table.html#Table.convert_unicode_to_bytestring) is currently implemented as
```
self._convert_string_dtype('S', 'U', python3_only)
```
but I think it should be
```
self._convert_string_dtype('U', 'S', python3_only)
```
?

It doesn't have any tests, so I'm not 100% sure.

I noticed this in https://github.com/astropy/astroquery/pull/41 where I'm struggling to write string data that contains the character `\u2212` to FITS via an Astropy table in Python 3 ... can anyone give an example how to do that or maybe even add this as a unit test?