Specifically these tests are to prevent a regression which has appeared a few times as we've been iterating on the `options` stuff. The problem is that the `:serializer` option specified further up in a serializer hierarchy gets passed down to lower associations.

Let's say we had the following: a `Category` class which `has_many :posts`, and then the `CategorySerializer` specifies a custom serializer for it's posts, `has_many :posts, serializer: FancyPostSerializer`. The `Category` and list of associated `Post`s are serialized as expected, but while serializing a `Post` we also need to serialize its `Comments`. The `serializer: FancyPostSerializer` option was correctly passed to the `CategorySerializer#posts` association, but also incorrectly propagated down to the `FancyPostSerializer#comments` association.

We don't currently have a fixture which allows such a hierarchy and options to expose this. Should I add the `Category` fixture back?