Fix for Rails 8.0.0

[tomukas]

Original code doesnt work well with Rails 8.0.0

For example:
in ruby console: User.all.to_json(only: [:id, :name])
Returns error can't modify frozen Hash: {:only=>[:id, :name]} (FrozenError)

This PR will fix it.

[scpike]

I can confirm this patch is necessary for Rails 8 compatibility. The underlying change is here: rails/rails#52826 (review).

This seems like a fine solution, it's similar to what active_model_serializers did at https://github.com/rails-api/active_model_serializers/pull/2482/files