We want to describe that `ArticleDecorator` responds to `parent_categories` and `related_categories`.

When describing `parent_categories` we want to assert that the collection returned has been decorated with `CategoryDecorator`.

When describing `related_categories` we want to assert that the collection returned has been decorated with `CategoryDecorator` and each item has been decorated with `CategoryContentDecorator`.

The fact that each decorator references  `object` tells us that the object to be decorated only needs to quack a certain way, it can be any type.