Ensure resolveBEMNode preserves key prop

[jednano]

Had issues on a different project, so I'm not sure if this is an issue or not, but write a test anyway to be sure.

Something to this effect:

describe('resolveBEMNode', () => {
    it('preserves a key prop', () => {
        expect(
            resolveBEMNode(<div key="foo" />).key
        ).toEqual('foo');
    });

    it('transfers `key` property if present', () => {
        const renderer = createRenderer();
        const Component = () => resolveBEMNode(<div key="foo" block="bar" />);

        renderer.render(<Component />);
        const component = renderer.getRenderOutput();

        expect(component.key).toBe('foo');
    });

    it('does not add superfluous `key` property if not present', () => {
        const renderer = createRenderer();
        const Component = () => resolveBEMNode(<div block="bar" />);

        renderer.render(<Component />);
        const component = renderer.getRenderOutput();

        expect(component.key).toBe(null);
    });
});

Note: use shallow rendering instead.