Adapt ShippingMethodsController to keep disallowing delete of shipping_methods associated with orders. Added controller tests to verify behaviour

spec/controllers/spree/admin/shipping_methods_controller_spec.rb

@@ -0,0 +1,31 @@
+require 'spec_helper'
+
+describe Spree::Admin::ShippingMethodsController, type: :controller do
+  include AuthenticationWorkflow
+
+  describe "shipping method not referenced by order" do
+    let(:shipping_method) { create(:shipping_method) }
+
+    scenario "is soft deleted" do
+      login_as_admin
+      expect(shipping_method.deleted_at).to be_nil
+
+      spree_delete :destroy, {"id" => shipping_method.id}
+
+      expect(shipping_method.reload.deleted_at).not_to be_nil
+    end
+  end
+
+  describe "shipping method referenced by order" do
+    let(:order) { create(:order_with_line_items) }
+
+    scenario "is not soft deleted" do
+      login_as_admin
+      expect(order.shipping_method.deleted_at).to be_nil
+
+      spree_delete :destroy, {"id" => order.shipping_method.id}
+
+      expect(order.shipping_method.reload.deleted_at).to be_nil
+    end
+  end
+end