Tests

While we have a good suite of tests already for this module, I'd like to add some more. Specifically, I'd like to look into [property based testing](https://elixir-lang.org/blog/2017/10/31/stream-data-property-based-testing-and-data-generation-for-elixir/).

I've been hearing about this a lot lately, and it looks like a good way to create an extensive test suite, and potentially uncover issues we've never considered.