Fix: https://github.com/rails/rails/commit/90616277e3d8fc46c9cf35d6a7470ff1ea0092f7#r168784389
Because the `depth` counter is inside `JSON::State` it can't be used
concurrently, and in case of a circular reference the counter may be
left at the max value.
The depth counter should be moved outside `JSON_Generator_State` and
into `struct generate_json_data`, but it's a larger refactor.
In the meantime, `JSON::Coder` calls `State#generate_new` so I changed
that method so that it first copy the state on the stack.
https://github.com/ruby/json/commit/aefa671eca
Étienne Barrié