don't mutate feature scheduled to refactoring

We are going to change the way we do calculations of offer/order totals

Author: porbas

ecommerce/pricing/.mutant.yml

@@ -16,3 +16,4 @@ matcher:
     - Pricing::Offer#remove_free_product
     - Pricing::Offer::List#set_free
     - Pricing::Offer::List#restore_nonfree
+    - Pricing::Offer::List#sub_amounts_total