[jss-nested][jss-expand][jss-observable][jss-function-values] Dynamic values in nested structures

With JSS of:

```css
{
alpha: {
'& div': observable
}
}
```

The first application will correctly use the value of the `observable`; however, when `observable` changes e.g. `{ height: '123' }`, the result never updates.

The observable is being monitored and the styleRule updated per [`onProcessRule`](https://github.com/cssinjs/jss/blob/master/packages/jss/src/plugins/observables.js#L38) as expected, however the `styleRule.prop` does not overwrite the prior value of the `& div` in the ``.

Noting related issues #735 #737