Fix missing method error when removing keys from a class

This PR fixes an issue we experienced calling `set_collection_name` on a model which had custom validations (e.g. `validates :foo; def foo; something; end`).

It would result in an error like this:

```
lib/mongo_mapper/plugins/keys.rb:250:in `block in remove_validations_for': undefined method `attributes' for :foo:Symbol (NoMethodError)
```