`build_longer_spec()` should transform before casting

[DavisVaughan]

This would match the rectangling functions and I think it generally makes more sense, as you use the transform to do the hard work, then the cast to validate your assumptions.

tidyr/R/pivot-long.R

Lines 317 to 328 in a63a537

	# optionally, cast variables generated from columns
	cast_cols <- intersect(names(names), names(names_ptypes))
	for (col in cast_cols) {
	names[[col]] <- vec_cast(names[[col]], names_ptypes[[col]])
	}
	# transform cols
	coerce_cols <- intersect(names(names), names(names_transform))
	for (col in coerce_cols) {
	f <- as_function(names_transform[[col]])
	names[[col]] <- f(names[[col]])
	}