[jss-isolate] isolate nested selectors with addRule

It looks like that isolation does not work for nested selectors of rules, added by `addRule/addRules`

```js
sheet.addRule('root', {
'& > *': {
isolate: true
}
})
```

It doesn't work:
https://www.webpackbin.com/bins/-Ky6_96xmo8v8B4ob6UK

Works:
https://www.webpackbin.com/bins/-Ky6UVkomYxYn8mUF4cu