fix: Avoid JSON dump on Rails.application (#6198)

Rails enhances `Object` with `#as_json` that either uses `#to_hash`, or
builds Hash of instance variables to their values. During boot process
Sidekiq sends debug event with context that includes `:reloader`. As
Reloader class has `@app = Rails.application` that causes stak level too
deep one-off failure.

lib/sidekiq/rails.rb

@@ -20,6 +20,10 @@ def call
       def inspect
         "#<Sidekiq::Rails::Reloader @app=#{@app.class.name}>"
       end
+
+      def to_hash
+        { app: @app.class.name }
+      end
     end
 
     # By including the Options module, we allow AJs to directly control sidekiq features