title: Symfony 4 Single-Text DateTime Widget no longer accepting yyyy-MM-dd string
body: <p>I'm sending a POST with the string 1980-01-02 as birth.</p>
  
  <p>My controller is verifying it using a form with this config.</p>
  
  <pre><code>  -&gt;add('birth', DateTimeType::class, array(
                  'required' =&gt; false,
                  'widget' =&gt; 'single_text', // so the API can send '2012-12-30'
                  'attr' =&gt; ['date-widget' =&gt; 'single_text', 'date_format' =&gt; 'yyyy-MM-dd']
              ));
  </code></pre>
  
  <p>This code was working on SF3, and maybe even on SF 4.1</p>
  
  <p>Now I'm getting this error. <code>Unable to reverse value for property path "birth": The date "1980-01-02" is not a valid date.</code></p>
  
  <p>Any idea how to solve it?
  Cheers</p>