Fix support for multi-select fields when using a datasource

Resolve #152.
It turns into a `1`, which is not even worth diving into. I thought about checking that it's `1`, but since that's not _valid_, testing against it seems wrong. Anyway, I figured it would be best to just check that when `multiple` isn't set, the values don't save, and when we add the `multiple` argument, the values do save.