title: React Final Form. How to set field value from component state
body: <p>I'm trying to add a date picker to my forum.
  Problem is the date picker only works through the component state.
  meaning that the picked values go into the state.</p>
  
  <p>I'm trying to force a final form field to use that state value.
  But I can't seem to figure it out.</p>
  
  <p>Basically trying to to this:</p>
  
  <pre><code>&lt;Field name="field1" value={this.state.date}/&gt;
  </code></pre>