Add before action to handle option type params

As reported in solidusio#5751, the new admin UI has a bug with the option type
selectors: only one option type can be added.

We narrowed this down to an issue handling option type params in the new
admin products controller. We noticed legacy controller contains a
before action to handle the incoming option type params, but the new one
did not. Re-adding the before action solves the bug. This is required
because the form input for option types contains comma separated values
within a string:

`<input value="1,2"type="hidden">`

Co-authored-by: An Stewart <[email protected]>

admin/app/controllers/solidus_admin/products_controller.rb

@@ -11,6 +11,8 @@ class ProductsController < SolidusAdmin::BaseController
     search_scope(:in_stock) { _1.where(id: Spree::Variant.in_stock.distinct.select(:product_id)) }
     search_scope(:out_of_stock) { _1.where.not(id: Spree::Variant.in_stock.distinct.select(:product_id)) }
 
+    before_action :split_params, only: [:update]
+
     def index
       products = apply_search_to(
         Spree::Product.includes(:master, :variants),
@@ -98,5 +100,14 @@ def activate
       flash[:notice] = t('.success')
       redirect_to products_path, status: :see_other
     end
+
+    def split_params
+      if params[:product][:taxon_ids].present?
+        params[:product][:taxon_ids] = params[:product][:taxon_ids].split(',')
+      end
+      if params[:product][:option_type_ids].present?
+        params[:product][:option_type_ids] = params[:product][:option_type_ids].split(',')
+      end
+    end
   end
 end