Fix changing config in subclass having global effect

[janko]

Purpose

Overriding configuration such as adapter in subclasses currently affects all serializers on 0-10-stable.

ActiveModelSerializers.config.adapter = :json_api

class FooSerializer < ActiveModel::Serializer
  config.adapter = :attributes
end

ActiveModelSerializers.config.adapter #=> :attributes

Changes

We make copy of configuration object on subclassing, just like was done in rails/rails#53970.

Related GitHub issues

This was a regression after removing ActiveSupport::Configurable in #2492.