Add autosave: true to Shipping Rate taxes

Shipping Rate taxes are something we want to be able to modify in the
order updater without having to update the order graph one by one.

Author: mamhoff

core/app/models/spree/shipping_rate.rb

@@ -12,7 +12,8 @@ class ShippingRate < Spree::Base
              class_name: "Spree::ShippingRateTax",
              foreign_key: "shipping_rate_id",
              inverse_of: :shipping_rate,
-             dependent: :destroy
+             dependent: :destroy,
+             autosave: true
 
     delegate :order, :currency, to: :shipment
     delegate :name, :tax_category, :tax_category_id, to: :shipping_method

core/spec/models/spree/shipping_rate_tax_spec.rb

@@ -75,5 +75,19 @@ module Spree
         end
       end
     end
+
+    describe "autosaving when order is saved" do
+      let(:tax_rate) { create(:tax_rate) }
+      let(:shipping_rate) { create(:shipping_rate) }
+      let(:shipment) { shipping_rate.shipment }
+
+      it "works" do
+        shipping_rate.taxes.new(
+          amount: -2,
+          tax_rate: tax_rate
+        )
+        expect { shipment.save! }.to change { Spree::ShippingRateTax.count }
+      end
+    end
   end
 end