def to_json(state = nil, *)
            state = State.from_state(state)
            state.check_max_nesting
            json_transform(state)
          end
Returns a JSON string containing a JSON object, that is unparsed from
this Hash instance.
_state_ is a JSON::State object, that can also be used to configure the
produced JSON string output further.
_depth_ is used to find out nesting depth, to indent accordingly.