Fix bug in Order#deliver_order_confirmation_email

When this method was re-introduced in solidusio#3485 with the
purpose of adding a deprecation warning, the reintroduced method had a
bug where `order` was being called on the order (instead of self).

Co-authored-by: Adam Mueller <adam@super.gd>

core/app/models/spree/order.rb

@@ -456,8 +456,8 @@ def deliver_order_confirmation_email
         "Spree::MailerSubscriber#order_finalized.",
         caller(1)
 
-      Spree::Config.order_mailer_class.confirm_email(order).deliver_later
-      order.update_column(:confirmation_delivered, true)
+      Spree::Config.order_mailer_class.confirm_email(self).deliver_later
+      update_column(:confirmation_delivered, true)
     end
 
     # Helper methods for checkout steps

core/spec/models/spree/order_spec.rb

@@ -62,6 +62,19 @@
     end
   end
 
+  describe '#deliver_order_confirmation_mailer' do
+    it 'sends an email' do
+      expect(Spree::Config.order_mailer_class).to receive(:confirm_email).and_call_original
+      order.deliver_order_confirmation_email
+    end
+
+    it 'marks the order as confirmation_delivered' do
+      expect do
+        order.deliver_order_confirmation_email
+      end.to change(order, :confirmation_delivered).to true
+    end
+  end
+
   context '#store' do
     it { is_expected.to respond_to(:store) }