Ensure cartons find soft deleted shipping methods

Even if a shipping method is soft deleted, cartons should still be able to find it. If they can't, the #tracking_url method blows up when it tries to call #build_tracking_url on a nil shipping method.

core/app/models/spree/carton.rb

@@ -3,7 +3,7 @@
 class Spree::Carton < Spree::Base
   belongs_to :address, class_name: 'Spree::Address', optional: true
   belongs_to :stock_location, class_name: 'Spree::StockLocation', inverse_of: :cartons, optional: true
-  belongs_to :shipping_method, class_name: 'Spree::ShippingMethod', inverse_of: :cartons, optional: true
+  belongs_to :shipping_method, -> { with_discarded }, class_name: 'Spree::ShippingMethod', inverse_of: :cartons, optional: true
 
   has_many :inventory_units, class_name: "Spree::InventoryUnit", inverse_of: :carton, dependent: :nullify
   has_many :orders, -> { distinct }, through: :inventory_units

core/spec/models/spree/carton_spec.rb

@@ -5,6 +5,14 @@
 RSpec.describe Spree::Carton do
   let(:carton) { create(:carton) }
 
+  describe 'shipping method' do
+    it 'returns soft deleted shipping method' do
+      carton = create(:carton)
+      carton.shipping_method.discard
+      expect(carton.reload.shipping_method).to be_present
+    end
+  end
+
   describe "#create" do
     subject { carton }