title: Django forms.DateInput does not apply the attributes given in attrs field
body: <p>Placeholder, class not getting set when tried to apply through the django's attrs specifier for <a href="https://docs.djangoproject.com/en/dev/ref/forms/widgets/#dateinput" rel="noreferrer">forms.DateInput</a></p>
  
  <p>The form is a <a href="https://docs.djangoproject.com/en/dev/topics/forms/modelforms/" rel="noreferrer">ModelForm</a>.</p>
  
  <p>And according to the <a href="https://docs.djangoproject.com/en/dev/ref/forms/widgets/#dateinput" rel="noreferrer">docs</a></p>
  
  <blockquote>
    <p>Takes same arguments as TextInput, with one more optional argument:</p>
  </blockquote>
  
  <p>Here is the code :</p>
  
  <pre><code>widgets = {
     'my_date_field': forms.DateInput(format=('%d-%m-%Y'), 
                      attrs={'class':'myDateClass', 
                             'placeholder':'Select a date'}
                      )
  }
  </code></pre>
  
  <p>The same is applied for a <code>forms.TextInput</code> and it works just fine.</p>
  
  <p>What am I missing here?</p>
  
  <p>Just anybody wants a full class code :</p>
  
  <pre><code>class trademark_form(ModelForm):
      my_date_field = DateField(input_formats=['%d-%m-%Y'])
      class Meta:
          model = myModel
  
          widgets = {
                     'my_date_field': forms.DateInput(format=('%d-%m-%Y'), attrs={'class':'myDateClass', 'placeholder':'Select a date'}),
                     'field1': forms.TextInput(attrs={'class':'textInputClass', 'placeholder':'Enter a Value..'}),
                     'field2': forms.TextInput(attrs={'class':'textInputClass', 'placeholder':'Enter a Value..', 'readonly':'readonly', 'value':10}),
                     'desc': forms.Textarea(attrs={'class':'textAreaInputClass', 'placeholder':'Enter desc', 'rows':5}),
  
                 }
          exclude = ('my_valid_field')
  </code></pre>
  
  <p>The generated HTML for the field, <code>my_date_field</code> :</p>
  
  <pre><code>&lt;input type="text" id="id_my_date_field" name="my_date_field"&gt;
  </code></pre>
  
  <p>The generated HTML for the field, <code>field1</code> :</p>
  
  <pre><code>&lt;input type="text" name="field1" class="textInputClass" placeholder="Enter a Value.." id="id_field1"&gt;
  </code></pre>