Fix transient failure in TodoService spec

For reasons unknown, the lack of `and_call_original` on this message
expectation was causing a transient failure depending on test order. We
narrowed it down using `rspec --bisect` to the following order:

    spec/requests/openid_connect_spec.rb:123 spec/services/todo_service_spec.rb:751

spec/services/todo_service_spec.rb

@@ -752,7 +752,7 @@ describe TodoService, services: true do
     issue = create(:issue, project: project, assignee: john_doe, author: author, description: mentions)
 
     expect(john_doe.todos_pending_count).to eq(0)
-    expect(john_doe).to receive(:update_todos_count_cache)
+    expect(john_doe).to receive(:update_todos_count_cache).and_call_original
 
     service.new_issue(issue, author)