`ChoicesFilterState` loses data class

`ChoicesFilterState` is used for all data classes if the number of unique values in the data is less than the option.
Selected values are actually stored as character strings.`$get_call` restores that class but `get_state` does not. This is not a problem yet but will be once we start saving and reloading `teal_slice`s.

Ensure that the selected values stored in `private$teal_slice` are of the appropriate class.