dia.CellView: `removeAttr()` does not remove the attribute from the root element

[kumilingus]

element.attr('root/test', true);
element.removeAttr('root/test');
// element.findView(paper).el.getAttribute('test') === 'true';

Workaround:

element.attr('root/test', null);
// element.findView(paper).el.hasAttribute('test') === false;