Cannot find descendants from .parent()

### Current behavior
```javascript
describe('Parent test', () => {
it('should return a parent that can find things', () => {
class MockComponent extends React.Component {
render() {
return (

);
}
}

const wrapper = shallow();

// The following passes on v3
expect(wrapper.find('.foo').parents().find('.bar')).toExist();
// The following fails on v3 but passes on v2
expect(wrapper.find('.foo').parent().find('.bar')).toExist();
});
});
```

### Expected behavior

The following use case should pass:
```javascript
expect(wrapper.find('.foo').parent().find('.bar')).toExist();
```

### Your environment

#### API

- [x] shallow
- [ ] mount
- [ ] render

#### Version

| library | version
| ------------------- | -------
| enzyme | 3.7.0
| react | 16.6.0, 16.5.0
| react-dom | 16.6.0, 16.5.0
| react-test-renderer | 16.6.0, 16.5.0
| adapter (below) |

#### Adapter

- [x] enzyme-adapter-react-16
- [ ] enzyme-adapter-react-16.3
- [ ] enzyme-adapter-react-16.2
- [ ] enzyme-adapter-react-16.1
- [ ] enzyme-adapter-react-15
- [ ] enzyme-adapter-react-15.4
- [ ] enzyme-adapter-react-14
- [ ] enzyme-adapter-react-13
- [ ] enzyme-adapter-react-helper
- [ ] others ( )

Start by reproducing the provided shallow-rendering test with enzyme-adapter-react-16, focusing on the difference between parents().find('.bar') and parent().find('.bar'). Trace the traversal behavior for parent() and find(); done means the stated parent().find('.bar') assertion passes for the shown component.

Written by the indexing model from the issue text.