Support aliasing position column

To support migrating projects from acts_as_list, the ranker should access the column value through the model's attributes, and not the accessor method which should be kept free for aliasing.

Ranker works with the internal column values in all other cases, I believe it should do so with the `rank` and `rank_at` methods too. This frees up the accessor method for aliasing to represent the external position value.

The added spec shows how this small change could be useful for existing models that use "position" as the column name and accessor method.