**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`

if (props.value != null) {
          value = props.value + '';
        } else {
          value = optionChildren;
        }
        selected = false;
        if (Array.isArray(selectValue)) {
          
          for (let j = 0; j < selectValue.length; j++) {
            if ('' + selectValue[j] === value) {
              selected = true;
              break;
            }
          }
        } else {
          selected = '' + selectValue === value;
        }
        props = Object.assign(
          {
            selected: undefined,
            children: undefined,
          },
          props,
          {
            selected: selected,
            children: optionChildren,
          },
        );