title: Field in the model does not change in django app
body: <p>I'm working on a django 1.4.5. I added to the model, select the size of T-shirts.</p>
  
  <p><strong>Field in my model</strong></p>
  
  <pre><code>tshirt_size = models.CharField(choices=TSHIRT_SIZE_CHOICES, default="s", blank=True, null=True, max_length=24)
  </code></pre>
  
  <p><strong>My form</strong></p>
  
  <pre><code>class SubscriberForm(forms.ModelForm):
      class Meta():
          model = Subscriber
          exclude = ['event', 'is_active']
          widgets = {
              'name': TextInput(),
              'last_name': TextInput(),
              'email': TextInput(),
              'tshirt_size': Select(choices=TSHIRT_SIZE_CHOICES)
          }
  </code></pre>
  
  <p>In view, I get data in this way:</p>
  
  <pre><code>tshirt_size = request.POST.get('tshirt_size')
  </code></pre>
  
  <p><strong>Part of html code</strong></p>
  
  <pre><code>&lt;label for="id_tshirt_size"&gt;T-Shirt Size (Unisex):&lt;/label&gt;
  {{ form.tshirt_size }}
  </code></pre>
  
  <p>And when I execute save on form I get in admin panel None value for tshirt_size.</p>