This helper no longer makes sense as a separate method. Instead I'll
just have `deserialize` call `cast` by default. This led to a random
infinite loop in the `JSON` pg type, when it called `super` from
`deserialize`. Not really a great way to fix that other than not calling
super, or continuing to have the separate method, which makes the public
API differ from what we say it is.

This predicate is only used in `query_attribute`, and is relatively easy
to remove without adding a bunch of is a checks.

The type code is actually quite accessible, and I'm planning to
encourage people to look at the files in the `type` folder to learn more
about how it works. This will help reduce the noise from code that is
less about type casting, and more about random AR nonsense.