**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.firstChild.getAttribute('stroke-dasharray')).toBe('10 10');
        },
      );
    });
    describe('unknown attributes', function() {
      itRenders('unknown attributes', async render => {
        const e = await render(<div foo="bar" />);
        expect(e.getAttribute('foo')).toBe('bar');
      });
      itRenders('unknown data- attributes', async render => {
        const e = await render(<div data-foo="bar" />);
        expect(e.getAttribute('data-foo')).toBe('bar');
      });
      itRenders('badly cased reserved attributes', async render => {
        const e = await render(<div CHILDREN="5" />, 1);
        expect(e.getAttribute('CHILDREN')).toBe('5');
      });
      itRenders('"data" attribute', async render => {
        
        const e = await render(<object data="hello" />);
        expect(e.getAttribute('data')).toBe('hello');
      });
      itRenders('no unknown data- attributes with null value', async render => {
        const e = await render(<div data-foo={null} />);
        expect(e.hasAttribute('data-foo')).toBe(false);
      });
      itRenders('unknown data- attributes with casing', async render => {
        const e = await render(<div data-fooBar="true" />, 1);
        expect(e.getAttribute('data-foobar')).toBe('true');