**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)
const shallowRenderer = createRenderer();
    
    
    shallowRenderer.render(
      React.createElement(SimpleComponent, initialProp),
      initialContext,
    );
    expect(getDerivedStateFromPropsParams).toEqual([
      [initialProp, initialState],
    ]);
    expect(componentDidUpdateParams).toEqual([]);
    expect(shouldComponentUpdateParams).toEqual([]);
    
    shallowRenderer.render(
      React.createElement(SimpleComponent, updatedProp),
      updatedContext,
    );
    expect(getDerivedStateFromPropsParams).toEqual([
      [initialProp, initialState],
      [updatedProp, initialState],
    ]);
    expect(shouldComponentUpdateParams).toEqual([
      updatedProp,
      initialState,
      updatedContext,
    ]);
    expect(componentDidUpdateParams).toEqual([]);
  });
  it('can shallowly render components with ref as function', () => {
    class SimpleComponent extends React.Component {