This is yet another behavior on which the various implementations differed, but the C implementation used to call to_json on String subclasses used as keys.
This was optimized out in e125072130229e54a651f7b11d7d5a782ae7fb65 but there is an Active Support test case for it, so it's best to make all 3 implementation respect this behavior.
Added by byroot (Jean Boussier) about 1 year ago
JSON.generate: call to_json on String subclasses
Fix: https://github.com/ruby/json/issues/667
This is yet another behavior on which the various implementations
differed, but the C implementation used to call
to_jsonon Stringsubclasses used as keys.
This was optimized out in e125072130229e54a651f7b11d7d5a782ae7fb65
but there is an Active Support test case for it, so it's best to
make all 3 implementation respect this behavior.