Fix for #321 Problem after Laravel upgrade 4.2.8 to 4.2.9

There is a change in Illuminate/Database/Eloquent/Builder::find and findMany between those versions that changes $this->query->where($this->model->getKeyName(), '=', $id); into $this->query->where($this->model->getQualifiedKeyName(), '=', $id);`.

`getQualifiedKeyName` prepends the key name with collection name and a dot, which I'm assuming MongoDB has a problem with. 

This changes the behavior so that MongoDB models return pure key name as qualified name.