falsy returns undefined when undefined is not in pool

The falsey pool parameter isn't working as intended. eg:

`
chance.falsy({pool: [null] });
`

That sometimes produces `undefined`, not sure if it's a bug - bug or the it is trying to also use undefined as a falsy value.

Start by locating the chance.falsy entry point and trace how its pool parameter is handled. Reproduce the example with pool set to [null], then determine whether undefined is included intentionally; done means the behavior matches the documented pool semantics and is covered by a regression test.

Written by the indexing model from the issue text.