title: MultipleHiddenInput renders only if with initial data
body: <pre><code>class MyForm(forms.Form):
      my_hidden_field = forms.MultipleChoiceField(widget=forms.MultipleHiddenInput, choices=(...))
  
  
  def my_view(request):
    form = MyForm(initial={'my_hidden_field': MyModel.objects.values_list('id', flat=True)})
  </code></pre>
  
  <p>With such code, if remove <code>initial</code> argument from call to <code>MyForm</code>, <code>my_hidden_field</code> will not be rendered in HTML, but if remove <code>MultipleHiddenInput</code> widget then, it will appear again.<br>
  How to make it being rendered properly?</p>