title: DatetimeType in Symfony Form: error validating sending a POST request manually
body: <p>I'm developing a REST API with Symfony and FOS Rest bundle. One of my entities has a date field. I'm also doing test with behat, my problem is when I try to test the post action of this action I always get a 400 error due to the validation of the Datetime field.
  Here's the field definition:</p>
  
  <pre><code>           -&gt;add(
                  'scheduledAt', DateTimeType::class, [
                  'label' =&gt; 'fields.scheduled_at',
                  'required' =&gt; false,
                  'format' =&gt; 'Y-m-d h:i'
              ])
  </code></pre>
  
  <p>And here's the behat test:</p>
  
  <pre><code>When I send a POST request to "/api/cases/1/actions" with body:
        """
        {
            "comments": "Comments 3",
            "action": 1,
            "scheduledAt": "2017-01-01 17:00:00"
        }
        """
  </code></pre>
  
  <p>This always end up with this 400 response</p>
  
  <pre><code>{
  "code":400,
  "message":"Validation Failed",
  "errors":{"children":{"comments":[],"scheduledAt":{"errors":["This value is not valid."],"children":{"date":{"children":{"year":[],"month":[],"day":[]}},"time":{"children":{"hour":[],"minute":[]}}}},"action":[]}}}
  </code></pre>