Custom fields are instantiated on every access, leading to very unexpected behaviours

Whenever a custom field (demongoized) is accessed, a new object is created - leading to changes not being persisted and performance overhead.

This closed ticket from 2015 addresses the same issue, but the issue is still unchanged 5 years later: https://jira.mongodb.org/browse/MONGOID-3341

IMO this is an extremely weird, nearly useless implementation and highly unexpected.

We're writing our own custom field implementation to work around it, but I believe this should be changed to work like any other document field.