Rails 4 compatibility: Tweaks to make it work with active model?

I'm working on updating an app to Rails4 with AR and MongoMapper. Since both work with ActiveModel I thought they should both work with just including the ActiveModel module. I had to make these tweaks to get it to work in both (in my app). However, some of the tests in this gem are failing. 

I wanted to run this by you and maybe you could give me some insight into why they are failing? Or if you would want me to do this some other way?