Add support for to_json

[bonekost]

This PR fixes a regression that I noticed in ActiveRecord::Base#as_json return value. (same as #630)

Description

If you enable attribute_methods plugin, mobility patches ActiveRecord::Base#attributes method to include translated attributes as wekk. The same change used to apply to ActiveRecord::Base#as_json too.

But the internal implementation of serialization changed slightly in Rails 7.0 to improve the performance.

As a result, as_json method no longer includes translated attributes.

Solution

I added an override of attribute_names_for_serialization method into the plugin to make it include the missing translated fields into serialized_attributes hash which is used by as_json later.

attribute_names_for_serialization would be never be called by Rails 6.x or lower, but I still added a safe guard against calling super.

[ollym]

@shioyama this is a rebase + changelog for #630 as that PR seemed to stop