2596 Respect STI on AA::Comments

![screen shot 2013-11-02 at 5 45 34 pm](https://f.cloud.github.com/assets/379894/1460030/0093a6ce-440a-11e3-853e-d3f902a590ae.png)

Fix [2596](https://github.com/gregbell/active_admin/issues/2596)