This will break code for people who have already set `Timezone.default_dst`.

I think this is something we should fix, but we should either default our `dst` variable to `Timezone.default_dst` instead of `nil` or just not pass it along if it is `nil`

I think I prefer not passing `dst` along if `nil`. That way we're providing the default `period_for_local` behavior even if that changes.