When setting boolean attributes on Web Components
`<x-search somebool name={this.props.name} />`
they get rendered as attribute/value pairs instead
`<x-search somebool="true" name={this.props.name}>`

This causes problems with some AMP components, for eg. `<amp-iframe />` which has a `resizable` attribute that gets rendered as `resizable="true"`. This results in non valid AMP content and developers crying with blood tears on their keyboards :)
> The attribute 'resizable' in tag 'amp-iframe' is set to the invalid value 'true'. (see https://www.ampproject.org/docs/reference/components/amp-iframe)

Is there any reason for the current behaviour? Will changing this break something else?
<div is="custom-element" className="test" />,
          render === clientRenderOnServerString ? 1 : 0,
        );
        expect(e.getAttribute('className')).toBe(null);
        expect(e.getAttribute('class')).toBe('test');
      } else {
        const e = await render(<div is="custom-element" className="test" />, 0);
        expect(e.getAttribute('className')).toBe('test');
      }
    });
    itRenders('htmlFor attribute on custom elements', async render => {
      const e = await render(<div is="custom-element" htmlFor="test" />);
      expect(e.getAttribute('htmlFor')).toBe('test');
      expect(e.getAttribute('for')).toBe(null);
    });
    itRenders('for attribute on custom elements', async render => {
      const e = await render(<div is="custom-element" for="test" />);
      expect(e.getAttribute('htmlFor')).toBe(null);
      expect(e.getAttribute('for')).toBe('test');
    });
    itRenders('unknown attributes for custom elements', async render => {
      const e = await render(<custom-element foo="bar" />);
      expect(e.getAttribute('foo')).toBe('bar');
    });
    itRenders('unknown `on*` attributes for custom elements', async render => {
      const e = await render(<custom-element onunknown="bar" />);
      expect(e.getAttribute('onunknown')).toBe('bar');
    });
    itRenders('unknown boolean `true` attributes as strings', async render => {
      const e = await render(<custom-element foo={true} />);