Serialize additional properties at the end of the json

[delvedor]

In some cases, the order of the keys is important, it might be for readability or other reasons. With the current implementation, every additional property is added at the beginning of the json, making it very hard to guarantee the order of the keys if you don't know them in advance.
This pr fixes this and serialize additional/pattern properties at the end of a json string.

Not sure if we should consider this a breaking change, as usually people test their code with JSON.parse.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows Code of conduct

[mcollina]

lgtm, I think this is a bug