Handle soft deleted products when refreshing cache

Since cd3add960e Spree soft-deletes products and as such the models not
destroyed but updated setting a value for the `deleted_at` field.

This turns what used to be deletes into updates thus triggering AR's
update callbacks instead of the destroy ones. As a result, this bypasses
the logic to refresh the products cache on destroy and hits the
`after_save` callback.

Furthermore, since act_as_paranoid (the soft-delete gem Spree uses) uses
a default scope to avoid retrieving soft-deleted records we need to
purposefully fetch them in order to refresh the cache.

app/models/spree/classification_decorator.rb

@@ -1,14 +1,14 @@
 Spree::Classification.class_eval do
-  belongs_to :product, :class_name => "Spree::Product", touch: true
-  after_save :refresh_products_cache
+  belongs_to :product, class_name: "Spree::Product", touch: true
+
   before_destroy :dont_destroy_if_primary_taxon
   after_destroy :refresh_products_cache
-
+  after_save :refresh_products_cache
 
   private
 
   def refresh_products_cache
-    product.refresh_products_cache
+    Spree::Product.with_deleted.find(product_id).refresh_products_cache
   end
 
   def dont_destroy_if_primary_taxon