**Do you want to request a *feature* or report a *bug*?**

Bug

### Current Behaviour

 - `ReactDOMServer.renderToString` is mutating CSS Variable names that are inlined and in the `--camelCase` format into `--kebab-case`

### Expected Behaviour

 - `ReactDOMServer.renderToString` does not mutate the CSS Variable name, and keeps it in `--camelCase` format

### Demo

 - https://codesandbox.io/s/react-dom-server-debugging-28xsr

### Versions:

 - Issue appears in `react@16.8.6` &  `react-dom@16.8.6` 
 - Issue appears in `react@16.0.0` & `react-dom@16.0.0` 
 - (That's all I've tested so far)

### Other notes:

 - It's worth noting that `ReactDOM.render` is preserving the CSS Variable name and not mutating it. It only seems to only be an issue with `ReactDOMServer.renderToString`

let div = <div style={styles} />;
    const root = document.createElement('div');
    div = ReactDOM.render(div, root);
    expect(/style=".*"/.test(root.innerHTML)).toBe(true);
  });
  it('should not set style attribute when no styles exist', () => {
    const styles = {
      backgroundColor: null,
      display: null,
    };
    const div = <div style={styles} />;
    const html = ReactDOMServer.renderToString(div);
    expect(/style=/.test(html)).toBe(false);
  });
  it('should warn when using hyphenated style names', () => {
    class Comp extends React.Component {
      static displayName = 'Comp';
      render() {
        return <div style={{'background-color': 'crimson'}} />;
      }
    }
    const root = document.createElement('div');
    expect(() => ReactDOM.render(<Comp />, root)).toWarnDev(
      'Warning: Unsupported style property background-color. Did you mean backgroundColor?' +
        '\n    in div (at **)' +
        '\n    in Comp (at **)',
    );
  });
  it('should warn when updating hyphenated style names', () => {
    class Comp extends React.Component {