Fix #3395: observer not working with <React.StrictMode>

[urugator]

Fixes #3395

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[urugator]

I wonder whether instead of patching componentDidMount, we could just call forceUpdate in componentWillUnmount, so the component is immediately dirty (and therefore forced to re-render) in case it re-mounts. The doc states:

You should not call setState() in componentWillUnmount() because the component will never be re-rendered. Once a component instance is unmounted, it will never be mounted again.

But that's obviously no longer true...

[mweststrate]

Thanks for investigating @urugator! Looks great