Override in_stock? to prepare for Spree 2.0

lib/open_food_network/scope_variant_to_hub.rb

@@ -20,6 +20,14 @@ module OpenFoodNetwork
         Spree::Price.new(amount: price, currency: currency)
       end
 
+      # Old Spree has the same logic as here and doesn't need this override.
+      # But we need this to use VariantOverrides with Spree 2.0.
+      def in_stock?
+        return true unless Spree::Config[:track_inventory_levels]
+
+        on_demand || (count_on_hand > 0)
+      end
+
       def count_on_hand
         @variant_override.andand.count_on_hand || super
       end