**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`

);
        expect(e.style.color).toBe('');
        expect(e.style.width).toBe('30px');
      });
      itRenders('no null styles', async render => {
        const e = await render(<div style={{color: null, width: '30px'}} />);
        expect(e.style.color).toBe('');
        expect(e.style.width).toBe('30px');
      });
      itRenders('no empty styles', async render => {
        const e = await render(<div style={{color: null, width: null}} />);
        expect(e.style.color).toBe('');
        expect(e.style.width).toBe('');
        expect(e.hasAttribute('style')).toBe(false);
      });
      itRenders('unitless-number rules with prefixes', async render => {
        const {style} = await render(
          <div
            style={{
              lineClamp: 10,
              WebkitLineClamp: 10,
              MozFlexGrow: 10,
              msFlexGrow: 10,
              msGridRow: 10,
              msGridRowEnd: 10,
              msGridRowSpan: 10,
              msGridRowStart: 10,
              msGridColumn: 10,
              msGridColumnEnd: 10,
              msGridColumnSpan: 10,