title: Django DateField initial/default value fails validation
body: <p>models:</p>
  
  <pre><code>def deliver_after_default():
      return timezone.now().date()
  
  
  deliver_after = models.DateField(default=deliver_after_default,
      blank=True,
      help_text="Don't delivery email or physical card until after this "
      "date. Delivery on this date cannot be guaranteed."
  )
  </code></pre>
  
  <p>I have Django autocreating a ModelFormf or this model. In my template:</p>
  
  <pre><code>      &lt;input class="form-control" name="{{ order_form.deliver_after.html_name }}" value="{{ order_form.deliver_after.value }}" type="date" /&gt;
  </code></pre>
  
  <p><code>order_form.deliver_after.value</code> gets rendered as 'Nov. 25, 2015'. Trying to submit that value in a form causes validation to fail:</p>
  
  <pre><code>{"deliver_after": ["Enter a valid date."]}
  </code></pre>
  
  <p>Am I missing something? Shouldn't Django be outputting initial values that can pass valdiation?</p>