Syntax error when emitting to JSON from a YAML zero leading number value

Emitting a JSON file from a YAML with a leading zero value will give a `SyntaxError` when parsed in JSON.

YAML
``` yaml
error: 04
```
JSON
``` json
{"error": 04}
```

Not sure if the responsibility is on the user or the library. Not sure if it's easy but when emitting to JSON you could convert to a string if there is a leading zero number, as it is invalid in JSON.