[WIP] Calculate order-level taxes in default

Still need a way to figure out which rates should be applied to the
order.

core/app/models/spree/tax_calculator/default.rb

@@ -25,6 +25,7 @@ def initialize(order)
       def calculate
         Spree::Tax::OrderTax.new(
           order_id: order.id,
+          order_taxes: order_rates,
           line_item_taxes: line_item_rates,
           shipment_taxes: shipment_rates
         )
@@ -34,6 +35,23 @@ def calculate
 
       attr_reader :order
 
+      # Calculate the order-level taxes.
+      #
+      # @private
+      # @return [Array<Spree::Tax::ItemTax>] calculated taxes for the order
+      def order_rates
+        rates_for_order.map do |rate|
+          amount = rate.compute_amount(order)
+
+          Spree::Tax::ItemTax.new(
+            label: rate.adjustment_label(amount),
+            tax_rate: rate,
+            amount: amount,
+            included_in_price: rate.included_in_price
+          )
+        end
+      end
+
       # Calculate the taxes for line items.
       #
       # @private
@@ -76,6 +94,19 @@ def calculate_rates(item)
           )
         end
       end
+
+      # @private
+      # @return [Array<Spree::TaxRate>] rates that apply to an order
+      def rates_for_order
+        tax_category_ids = Set[
+          *@order.line_items.map(&:tax_category_id),
+          *@order.shipments.map(&:tax_category_id)
+        ]
+        rates = Spree::TaxRate.active.order_level.for_address(@order.tax_address)
+        rates.select do |rate|
+          tax_category_ids.intersect?(rate.tax_category_ids.to_set)
+        end
+      end
     end
   end
 end