Fix 2.7 warning for sidekiq_middleware_spec.rb
Related issue: #257438 (closed)
ActiveJob arguments have a special key for kwargs: https://github.com/rails/rails/blob/8e452c710d9f8bb7dec24407c0c3c5508e26ef01/activejob/lib/active_job/arguments.rb#L55
Let's use this key in this particular test since it's the intention