**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`

itRenders('simple numbers', async render => {
        const e = await render(<div width={30} />);
        expect(e.getAttribute('width')).toBe('30');
      });
      itRenders('simple strings', async render => {
        const e = await render(<div width={'30'} />);
        expect(e.getAttribute('width')).toBe('30');
      });
      itRenders('no string prop with true value', async render => {
        const e = await render(<a href={true} />, 1);
        expect(e.hasAttribute('href')).toBe(false);
      });
      itRenders('no string prop with false value', async render => {
        const e = await render(<a href={false} />, 1);
        expect(e.hasAttribute('href')).toBe(false);
      });
      itRenders('no string prop with null value', async render => {
        const e = await render(<div width={null} />);
        expect(e.hasAttribute('width')).toBe(false);
      });
      itRenders('no string prop with function value', async render => {
        const e = await render(<div width={function() {}} />, 1);
        expect(e.hasAttribute('width')).toBe(false);
      });
      itRenders('no string prop with symbol value', async render => {
        const e = await render(<div width={Symbol('foo')} />, 1);
        expect(e.hasAttribute('width')).toBe(false);
      });
    });
    describe('boolean properties', function() {