Caching doesn't seem to work correctly

Hey 👋🏼

First off, this is a great gem! I have enjoyed working with it.

I started caching my serializer using the `cached` macro. As I understand from the documentation, that would use the `cache_key` on the resource.

It does cache the first time I serialize the resource. Then I updated the resource and serialized it again. The second serialization uses the cached JSON, even though the resource has been updated.

Thanks,
Emil