Unmount and then wrapper.mount() again does not work properly

Enzyme: 2.8.2
React: 15.5.4

If we do wrapper.unmount(), and then wrapper.mount(), the component is not mounted properly. I can see an empty console output after mounting again.

Simple plain example as follows
```jsx
class Foo extends React.Component {
constructor(props) {
super(props);

}
render() {
return (

const wrapper = mount();

console.log("log before unmounting", wrapper.mount().debug()); //This logs the component properly

wrapper.unmount();

console.log("Log again after mounting", wrapper.mount().debug());//this logs empty component
```

Start by running the provided React 15.5.4 and Enzyme 2.8.2 reproduction using mount, wrapper.unmount(), and wrapper.mount(). Trace the wrapper mount and unmount entry points to find why the second debug() is empty. Done means the component renders correctly after unmounting and mounting again, with a regression test covering the sequence.

Written by the indexing model from the issue text.