title: ModelChoiceField widget attrs have no effect
body: <p>I have this model form that I have to render manually but the ModelChoiceField attributes don't have any effect. </p>
  
  <pre><code>class ExtendedUserForm(forms.ModelForm):
      favourite_provider = forms.ModelChoiceField(queryset=Provider.objects.all())
  
      class Meta:
          EXPERTISE_CHOICES = (
              ('N', 'Novice'),
              ('B', 'Beginner'),
              ('I', 'Intermediate'),
              ('E', 'Expert'),
          )
  
          model = ExtendedUser
          labels = {
              'expertise': 'Expertise',
              'favourite_provider': 'Favourite provider',
              'job_title': 'Job title',
              'job_place': 'Company/Institution',
          }
  
          widgets = {
              'expertise': forms.Select(choices=EXPERTISE_CHOICES, attrs={'class': 'form-control'}),
              'favourite_provider': forms.Select(attrs={'class': 'form-control'}),
              'job_title': forms.TextInput(attrs={'placeholder': 'Your job title', 'class': 'form-control'}),
              'job_place': forms.TextInput(attrs={'placeholder': 'Your company/institution', 'class': 'form-control'}),
          }
          exclude = ['user', 'history', 'favourite_services']
  </code></pre>
  
  <p>The rendered HTML is as follow:</p>
  
  <pre><code>&lt;div class="form-group"&gt;
      &lt;label&gt;&lt;label for="id_expertise"&gt;Expertise:&lt;/label&gt;&lt;/label&gt;
      &lt;select name="expertise" required id="id_expertise" class="form-control"&gt;
        &lt;option value="" selected&gt;---------&lt;/option&gt;
        &lt;option value="N"&gt;Novice&lt;/option&gt; 
        &lt;option value="B"&gt;Beginner&lt;/option&gt;
        &lt;option value="C"&gt;Competent&lt;/option&gt;
        &lt;option value="P"&gt;Proficient&lt;/option&gt;
        &lt;option value="E"&gt;Expert&lt;/option&gt;
      &lt;/select&gt;
    &lt;/div&gt;
  
  
   &lt;div class="form-group"&gt;
   &lt;label&gt;&lt;label for="id_favourite_provider"&gt;Favourite provider:&lt;/label&gt;&lt;/label&gt;
   &lt;select name="favourite_provider" required id="id_favourite_provider"&gt;
        &lt;option value="" selected&gt;---------&lt;/option&gt;
        &lt;option value="1"&gt;AWS&lt;/option&gt;
      &lt;/select&gt;
  </code></pre>
  
  <p>As you can see the second select doesn't have the class attribute despite that I precised that here <code>'favourite_provider': forms.Select(attrs={'class': 'form-control'})</code>. How to fix that ?</p>