Prevent getAttribute() from calling static methods

I hit an issue after upgrading to v2 that I could no longer access attributes that had the same name as static method declared on Jenssegers\Mongodb\Model (or its parents).

For example I had attributes called 'updated' and 'deleted'. When trying to access $model->updated, PHP threw a fatal error because `$relations = $this->$camelKey();` tried calling `\Illuminate\Database\Eloquent\Model::updated($callback)`.

This pull request helps mitigate the issue by ensuring only non-static methods are assumed to represent a embedded model.