BUG: CASDA query at 0 DEC exactly

Hi,

I've noticed that `astroquery.casda.Casda.query_region` fails if the sky coordinates are given at exactly 0 declination.

I'm not sure why, as the error message is not very descriptive, but it doesn't seem to be an integer vs float issue at least, and it also doesn't seem to be this particular region of sky, as a tiny deviation from 0 DEC does work:

This is using
```
astroquery.__version__
'0.4.10.dev9924'
```

Start at astroquery.casda.Casda.query_region and reproduce the failure with a coordinate at exactly 0 declination, then compare it with a tiny nonzero declination. Inspect the resulting error and the surrounding CASDA query behavior; done means the exact-zero case succeeds without breaking nearby coordinates.

Written by the indexing model from the issue text.