**Do you want to request a *feature* or report a *bug*?**
bug

**What is the current behavior?**
In normal rendering, you can memoize a class-based component: https://jsfiddle.net/586ea3cx/

With the shallow renderer, it seems like you can't: https://jsfiddle.net/odj217Lv/1/

This might be blocking https://github.com/airbnb/enzyme/pull/1914, in which tests are failing with `Cannot call a class as a function`. (it's tough to repro stuff with the shallow renderer)
return <div />;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<SomeComponent />);
    shallowRenderer.unmount();
    expect(componentWillUnmount).toBeCalled();
  });
  it('can shallow render to null', () => {
    class SomeComponent extends React.Component {
      render() {
        return null;
      }
    }
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SomeComponent />);
    expect(result).toBe(null);
  });
  it('can shallow render with a ref', () => {
    class SomeComponent extends React.Component {
      render() {
        return <div ref="hello" />;
      }
    }
    const shallowRenderer = createRenderer();
    
    shallowRenderer.render(<SomeComponent />);
  });
  it('lets you update shallowly rendered components', () => {
    class SomeComponent extends React.Component {