**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`

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @emails react-core
 */
'use strict';
const React = require('react');
const ReactDOM = require('react-dom');
const ReactDOMServer = require('react-dom/server');
describe('CSSPropertyOperations', () => {
  it('should automatically append `px` to relevant styles', () => {
    const styles = {
      left: 0,
      margin: 16,
      opacity: 0.5,
      padding: '4px',
    };
    const div = <div style={styles} />;
    const html = ReactDOMServer.renderToString(div);
    expect(html).toContain('"left:0;margin:16px;opacity:0.5;padding:4px"');
  });
  it('should trim values', () => {
    const styles = {
      left: '16 ',