Extend function property values are converted to strings

__Expected behavior:__
A clear and concise description of what you expected to happen.

```
const styles = {
linkCommon: {
color: () => "orange"
},
link: {
extend: "linkCommon"
}
};
```

The link should be orange.

__Describe the bug:__
A clear and concise description of what happend instead of the expected behavior.

The function is being converted to a string before being inserted into the DOM.


__Codesandbox link:__

Playground: https://codesandbox.io/embed/core-jss-playground-ef0j4

__Versions (please complete the following information):__
- jss: 10 alpha 18
- jss-preset-default: 10 alpha 18
- Browser Chrome 75
- OS: OX X

_Feel free to add any additional versions which you may think are relevant to the bug._

Not to be confused with https://github.com/cssinjs/jss/issues/479