Well, actually it is. The original regression was that passing a custom `:serializer` option would propagate down to associations. However, with these fixtures it won't expose the problem b/c `Comment` has no associations.

I recall now that this is why I added a new `Category` fixture, and gave it a custom `serializer: CustomPostSerializer` option, which would propagate down and be used to serializer the post's comments.

I think I need to bring back the extra fixtures to properly expose this. What do you think?