It seems problematic to allow arbitary non-lowercase attribute on HTML nodes. Because the server has no ability to keep them case sensitive (HTML isn't). So they will end up different on the client.

We should probably warn if the attribute is not a known one *and* is not lowercase.
expect(e.getAttribute('hidden')).toBe('');
      });
      
      
      
      itRenders('boolean prop with "" value', async render => {
        const e = await render(<div hidden="" />);
        expect(e.getAttribute('hidden')).toBe(null);
      });
      
      itRenders('boolean prop with string value', async render => {
        const e = await render(<div hidden="foo" />);
        expect(e.getAttribute('hidden')).toBe('');
      });
      
      itRenders('boolean prop with array value', async render => {
        const e = await render(<div hidden={['foo', 'bar']} />);
        expect(e.getAttribute('hidden')).toBe('');
      });
      
      itRenders('boolean prop with object value', async render => {
        const e = await render(<div hidden={{foo: 'bar'}} />);
        expect(e.getAttribute('hidden')).toBe('');
      });
      
      itRenders('boolean prop with non-zero number value', async render => {
        const e = await render(<div hidden={10} />);
        expect(e.getAttribute('hidden')).toBe('');
      });