Fix the `collection_for` method

That a pretty good example what happens if you skip the tests… The `collection_for` method should return the correct `Collection` class in both cases:

```ruby
DocumentModelMapper.collection_for :user # => UsersCollection
DocumentModelMapper.collection_for :user_session # => UserSessionsCollection
```

That will work now, the latter didn't worked before. Thanks to @unplugandplay for finding this.

But still, we need to rewrite this and I'm still not adding some tests ;-)