**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`

static displayName = 'Comp';
      render() {
        return <div style={this.props.style} />;
      }
    }
    const styles = {
      '-ms-transform': 'translate3d(0, 0, 0)',
      '-webkit-transform': 'translate3d(0, 0, 0)',
    };
    const root = document.createElement('div');
    ReactDOM.render(<Comp />, root);
    expect(() => ReactDOM.render(<Comp style={styles} />, root)).toWarnDev([
      'Warning: Unsupported style property -ms-transform. Did you mean msTransform?' +
        '\n    in div (at **)' +
        '\n    in Comp (at **)',
      'Warning: Unsupported style property -webkit-transform. Did you mean WebkitTransform?' +
        '\n    in div (at **)' +
        '\n    in Comp (at **)',
    ]);
  });
  it('warns when miscapitalizing vendored style names', () => {
    class Comp extends React.Component {