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