The docs say that it supports a string... But is seems to support js numbers too. And people are gonna pass in JS numbers unless it's forbidden. So to avoid an accidental behavior-breaking change in the future, I figured I'd add a test.

In semver, since you documented that functions require strings, I suppose it wouldn't require a major version bump if you disallowed js numbers as inputs. In practice, that feels kinda wrong.

Note I ran the tests locally and they passed, including my new test.