number_to_words: support for custom comma-separators is broken/incomplete

Discovered while refactoring the [`number_to_words` method](https://github.com/jaraco/inflect/blob/cd7142ff757b40bf93ad0e4389ab62988ee48b88/inflect/__init__.py#L3790-L3938), as part of reducing complexity of the package in #174:

Using a custom `comma` argument to `number_to_words` appears to be supported based on the docstrings, and that would match the corresponding documentation for the [`EN::Inflect` CPAN module](https://metacpan.org/pod/Lingua::EN::Inflect) that this library is based on.

However: in practice it seems that providing a `comma` argument (other than the default `,`) does not work as expected.

Read the number_to_words method in inflect/__init__.py, especially the implementation linked in the issue, and compare its behavior with the method's docstrings and the referenced Lingua::EN::Inflect documentation. Reproduce a call using a non-default comma argument, then add coverage demonstrating the documented separator behavior and confirming it works as intended.

Written by the indexing model from the issue text.