Observed this on react@16.0.0-beta.5

```js
import React from 'react';
import { renderToString } from 'react-dom/server';

const B = () => 'b';

console.log(renderToString(<div>{'a'}<B />{'c'}</div>));
```

This renders as `<div data-reactroot="">a<!-- -->bc</div>`. If you try to hydrate this you'll get the error: Text content did not match. Server: "bc" Client: "b".
itRenders('boolean prop with zero value', async render => {
        const e = await render(<div hidden={0} />);
        expect(e.getAttribute('hidden')).toBe(null);
      });
      itRenders('no boolean prop with null value', async render => {
        const e = await render(<div hidden={null} />);
        expect(e.hasAttribute('hidden')).toBe(false);
      });
    });
    describe('download property (combined boolean/string attribute)', function() {
      itRenders('download prop with true value', async render => {
        const e = await render(<a download={true} />);
        expect(e.getAttribute('download')).toBe('');
      });
      itRenders('download prop with false value', async render => {
        const e = await render(<a download={false} />);
        expect(e.getAttribute('download')).toBe(null);
      });
      itRenders('download prop with string value', async render => {
        const e = await render(<a download="myfile" />);
        expect(e.getAttribute('download')).toBe('myfile');
      });
      itRenders('download prop with string "false" value', async render => {
        const e = await render(<a download="false" />);
        expect(e.getAttribute('download')).toBe('false');
      });
      itRenders('download prop with string "true" value', async render => {
        const e = await render(<a download={'true'} />);
        expect(e.getAttribute('download')).toBe('true');
      });