Re-add matching products limiter to per_item calculator

As per #1526, related to #1524 and #1596.

core/app/models/spree/calculator/per_item.rb

@@ -11,7 +11,23 @@ def self.description
     def compute(object=nil)
       return 0 if object.nil?
       self.preferred_amount * object.line_items.reduce(0) do |sum, value|
-        sum + value.quantity
+        value_to_add = matching_products.include?(value.product) ? value.quantity : 0
+        sum + value_to_add
+      end
+    end
+
+    # Returns all products that match this calculator, but only if the calculator
+    # is attached to a promotion. If attached to a ShippingMethod, nil is returned.
+    def matching_products
+      # Regression check for #1596
+      # Calculator::PerItem can be used in two cases.
+      # The first is in a typical promotion, providing a discount per item of a particular item
+      # The second is a ShippingMethod, where it applies to an entire order
+      #
+      # Shipping methods do not have promotions attached, but promotions do
+      # Therefore we must check for promotions
+      if self.calculable.respond_to?(:promotion)
+        self.calculable.promotion.rules.map(&:products).flatten
       end
     end
   end