title: writeValue with invalid value in custom form control
body: <p>When I want to create custom form control in Angular I need to implement the <code>ControlValueAccessor</code> which defines <code>writeValue(value:any): void</code> function.</p>
  
  <p>Now imagine that the custom component only accepts strings which contains only numbers because of its inner working. It cannot accept nothing else as it cannot do anything meaningful with it. </p>
  
  <p>Questions when someone calls <code>formControl.setValue()</code> with invalid value:</p>
  
  <ul>
  <li>What should be the value of that <code>formControl</code> (<code>null</code> or invalid input)?</li>
  <li>Should <code>writeValue</code> call <code>onChange</code> callback with <code>null</code> in case of invalid input, so that Angular knows that the value was not accepted?</li>
  </ul>