sheet.addRule API does not work as documented

Hi, first thanks for great JSS tool.

I have a problem with ```sheet.addRule```. Following example for already attached ```sheet``` (based on API documentation at https://github.com/cssinjs/jss/blob/master/docs/js-api.md) does not work for me:

```
const rule = sheet.addRule({ padding: 20})
alert (rule.className)
```

1. ```rule.className``` is undefined
2. rule is not added to DOM

When I use ```sheet.addRule('xxx', { padding: 20}); alert (sheet.classes.xxx)```, all is working as expected.

I am using JSS ver. 9.0.0 and latest Chrome.

Thank you for clarification.