alias_method fails picking up alias_attribute

Fixing `alias_method': undefined method `amount' for class `Spree::ShippingRate' (NameError)

For now with just a method that returns the aliased attribute. We probably just want to use `cost` here though.

core/app/models/spree/shipping_rate.rb

@@ -17,9 +17,11 @@ class ShippingRate < Spree::Base
     delegate :order, :currency, to: :shipment
     delegate :name, :tax_category, :tax_category_id, to: :shipping_method
     delegate :code, to: :shipping_method, prefix: true
-    alias_attribute :amount, :cost
 
-    alias_method :total_before_tax, :amount
+    alias_attribute :amount, :cost
+    def total_before_tax
+      amount
+    end
 
     extend DisplayMoney
     money_methods :amount