Apply tax on discounted price

Author: cyu

app/models/spree/line_item_decorator.rb

@@ -1,6 +1,6 @@
 Spree::LineItem.class_eval do
   def tax_cloud_cache_key
-    key = "Spree::LineItem #{id}: #{quantity}x<#{variant.cache_key}>@#{price}#{currency}"
+    key = "Spree::LineItem #{id}: #{quantity}x<#{variant.cache_key}>@#{price}#{currency}promo_total<#{promo_total}>"
     if order.ship_address
       key << "shipped_to<#{order.ship_address.try(:cache_key)}>"
     elsif order.bill_address

app/models/spree/tax_cloud.rb

@@ -48,7 +48,7 @@ def self.cart_item_from_item(item, index)
           index:    index,
           item_id:  item.try(:variant).try(:sku).present? ? item.try(:variant).try(:sku) : "LineItem #{item.id}",
           tic:      (item.product.tax_cloud_tic || Spree::Config.taxcloud_default_product_tic),
-          price:    item.price,
+          price:    item.quantity == 0 ? item.price : (item.promo_total / item.quantity) + item.price,
           quantity: item.quantity
         )
       when Spree::Shipment