title: shouldComponentUpdate in function components
body: <p>I have a question regarding React's <code>shouldComponentUpdate</code> (when not overwritten). I do prefer pure, function components, but I am afraid that it updates every time, even though props/state did not change. 
  So I am considering using the PureComponent class instead. </p>
  
  <p>My question regarding that:
  Do function components have the same <code>shouldComponentUpdate</code> check as PureComponents? Or does it update every time?</p>