Fix compatibility with ActiveRecord 3.2.12

Currently adding roles to unsaved records fails

```ruby
User.create!(email: "x@y.com", password: "xyz", role_ids: [1,2])

ActiveRecord::RecordInvalid: Validation failed: Role assignments is invalid
``` 

Also the `should_save_association_when_saving_new_record` test fails.

This patch fixes it and makes `role_play` compatible with ActiveRecord 3.2.12.