ChoicesFilterState loses data class

[chlebowa]

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_slices.

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