**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={{
              fontFamily: 'Helvetica, arial',
              backgroundImage: 'url(foo;bar)',
              backgroundColor: 'blue;',
              color: 'red;   ',
            }}
          />
        );
      }
    }
    const root = document.createElement('div');
    expect(() => ReactDOM.render(<Comp />, root)).toWarnDev([
      "Warning: Style property values shouldn't contain a semicolon. " +
        'Try "backgroundColor: blue" instead.' +
        '\n    in div (at **)' +
        '\n    in Comp (at **)',
      "Warning: Style property values shouldn't contain a semicolon. " +
        'Try "color: red" instead.' +
        '\n    in div (at **)' +
        '\n    in Comp (at **)',
    ]);
  });
  it('should warn about style containing a NaN value', () => {
    class Comp extends React.Component {