Missing `vec_cast()` methods for haven_labelled_spss

[rubenarslan]

You added casting methods for haven_labelled in #522 but forgot about haven_labelled_spss. See below (this is in 2.3.1):

library(haven)
x <- labelled(1:5)
vctrs::vec_cast(x, double())
x <- labelled_spss(1:5)
vctrs::vec_cast(x, double())

[rubenarslan]

I don't understand vctrs well enough for a PR, but those are the missing functions breaking my package. Or maybe you don't intend to support this for labelled_spss? Looking at arithmetic, that certainly could get messy if you want to support missing values.

#' @export
vec_cast.double.haven_labelled_spss <- function(x, to, ...) vec_cast(vec_data(x), to)
#' @export
vec_cast.integer.haven_labelled_spss <- function(x, to, ...) vec_cast(vec_data(x), to)
#' @export
vec_cast.character.haven_labelled_spss <- function(x, to, ...) {
  if (is.character(x)) {
    vec_cast(vec_data(x), to, ...)
  } else {
    stop_incompatible_cast(x, to, ...)
  }
}