title: Django update model with unique field
body: <p>I am trying to allow users to edit a topic. Whenever I try to update the topic I get the following error:</p>
  
  <p>Title: Topic with this Title already exists.</p>
  
  <pre><code># views.py
  ...
  def post(self, request, title):
      topic = Topic.objects.all().get(title=title)
      form = self.form_class(request.POST)
      if (form.is_valid()):
          topic.update(
              title=form.cleaned_data('title'),
              tags=form.cleaned_data('tags'),
          )
          return redirect('/topics/' + topic.title)
      else:
          return render(request, self.template_name, {'topic': topic, 'form': form})
  
  # models.py
  class Topic(models.Model):
      ...
      title = models.CharField(max_length=256, unique=True)
      ...
  
  # forms.py
  class EditTopicForm(forms.ModelForm):
      class Meta:
          model = Topic
          fields = [
              'title',
              'tags',
          ]
          widgets = {
              'tags': TagWidget(),
          }
  </code></pre>