Stringify method works incorrectly

After `3.2.0` release the `stringify` is working incorrectly while using comments -> `{ keepWsc: true }`.
Tested in Kibana Vega editor (which uses `hjson.stringify`):

![vega_bug](https://user-images.githubusercontent.com/31325372/67201986-41141500-f410-11e9-8a82-6d08471c4a85.gif)

Seems it duplicates keys of comments.