wrapper.instance() throws error after wrapper.unmount()

When calling `wrapper.instance()` after `wrapper.unmount()` it throws `TypeError: component.getPublicInstance is not a function`

The use-case for accessing the React component instance after the unmount is to check that any possible teardown in `componentWillUnmount()` has been successful. In my particular use-case I want to check that an RxJS Subscription has been unsubscribed.

Code to reproduce:
```jsx
import React from 'react';
import Rx from 'rxjs';
import { mount } from 'enzyme';

class MyComponent extends React.Component {

constructor() {
super();
this.state = {
foo: 'initial'
};
}

componentDidMount() {
this.subscription = this.props.subject.subscribe(
value => this.setState({ foo: value })
);
}

componentWillUnmount() {
this.subscription.unsubscribe();
}

render() {
return

;
}
}

it('unsubscribes on unmount', () => {
const subject = new Rx.BehaviorSubject('new value');

const wrapper = mount(
,
);
expect(wrapper.instance()).toHaveProperty('subscription.closed', false);
wrapper.unmount();
expect(wrapper.instance()).toHaveProperty('subscription.closed', true);
});
```

Test output:
```
FAIL src/ui/memo/__tests__/AfterUnmount.test.js
● unsubscribes on unmount

TypeError: component.getPublicInstance is not a function

at WrapperComponent.getInstance (node_modules/enzyme/build/ReactWrapperComponent.js:81:32)
at ReactWrapper.instance (node_modules/enzyme/build/ReactWrapper.js:219:31)
at Object..it (src/ui/memo/__tests__/AfterUnmount.test.js:37:18)
at Promise.resolve.then.el (node_modules/p-map/index.js:42:16)
at process._tickCallback (internal/process/next_tick.js:103:7)
```

There is a workaround by using `wrapper.node` instead of `wrapper.instance()` in the last expect: `expect(wrapper.node).toHaveProperty('subscription.closed', true);`. But this feels dirty because `.node` is an undocumented feature.

Used versions:
enzyme: 2.8.2
jest: 20.0.3
react: 15.5.4
react-scripts: 1.0.5
rxjs: 5.4.0
enzyme-to-json: 1.5.1 (I don't think this affects this particular test, but I had it installed anyway)

Start with the failing test in src/ui/memo/__tests__/AfterUnmount.test.js and reproduce the error using wrapper.instance() after wrapper.unmount(). Compare the behavior with the documented wrapper.node workaround; done means instance() can inspect the component after unmount without throwing and the test verifies the subscription is closed.

Written by the indexing model from the issue text.