Improve test coverage

The current test coverage is around 50%, so we should work on improving that:

https://coveralls.io/r/astropy/pyregion

Of course, we shouldn't try and improve coverage of e.g. `kapteyn_celetial.py` since this is an external package (and see #43 for a suggestion of removing it in favor of using `astropy.coordinates`).

Start with the Coveralls report linked in the issue to identify uncovered project code. Exclude kapteyn_celetial.py as requested and review the discussion in #43 before choosing targets. Done means adding tests that materially improve coverage without covering that external package.

Written by the indexing model from the issue text.