No, this isn't a cache versioning issue. The content of the cached blob won't change. The issue here is that we can end up with either `nil`, `CACHED_NIL`, a `CollectionAssociation` (new with this change), a `CollectionProxy`, a record, or some other kind of `Association` in the instance variable cache.

Previously, we'd just return the contents of the instance variable (after unmapping `CACHED_NIL`), but now that we can have a `CollectionAssociation`, we need to distinguish that type and call `.reader` to get the associated `CollectionProxy`. `reflect_on_association(association_name)).collection?` doesn't distinguish between `CollectionAssociation` and `CollectionProxy`.