title: Django Update view saving but not updating
body: <p>I have made a working form to post reviews for restaurants.<br>
  When user is logged in he can see displayed under his post the edit button.<br>
  When I use the Updateview however, it doesn't edit, but instead creates a new line in which the new review appears (and that new line doesn't appear then on my webpage).<br>
  Here is the model:</p>
  
  <pre><code>class UserReview(models.Model):
      # Defining the possible grades
      Grade_1 = 1
      Grade_2 = 2
      Grade_3 = 3
      Grade_4 = 4
      Grade_5 = 5
      # All those grades will sit under Review_Grade to appear in choices
      Review_Grade = (
          (1, '1 - Not satisfied'),
          (2, '2 - Almost satisfied'),
          (3, '3 - Satisfied'),
          (4, '4 - Very satisfied'),
          (5, '5 - Exceptionally satisfied')
      )
      restaurant = models.ForeignKey(Restaurant, on_delete=models.CASCADE)
      user_review_grade = models.IntegerField(default=None, choices=Review_Grade) # default=None pour eviter d'avoir un bouton vide sur ma template
      user_review_comment = models.CharField(max_length=1500)
      posted_by = models.ForeignKey(User, on_delete=models.DO_NOTHING)
      def get_absolute_url(self):
          return reverse('restaurants:reviews', args=[self.id])
  
      def get_edit_url(self):
          return reverse('restaurants:edit-review', args=(self.id,))
  </code></pre>
  
  <p>Here is the update form (similar as review form):</p>
  
  <pre><code>class EditReviewForm(forms.ModelForm):
      class Meta:
          model = UserReview
          fields = [
              'restaurant',
              'user_review_grade',
              'user_review_comment'
          ]
          widgets = {
              'restaurant': forms.HiddenInput,
              'user_review_grade': forms.RadioSelect,
              'user_review_comment': forms.Textarea
          }
          labels = {
              'user_review_grade': 'Chose a satisfaction level:',
              'user_review_comment': 'And write your comments:'
          }
  </code></pre>
  
  <p>Here is the UpdateView:</p>
  
  <pre><code>class EditReview (LoginRequiredMixin, UpdateView):
      template_name = 'restaurants/review_edit.html'
      form_class = EditReviewForm
      model = UserReview
      slug = 'review'
      # Post the data into the DB
      def post(self, request, pk, *args, **kwargs):
          form = UserReviewForm(request.POST)
          restaurant = get_object_or_404(Restaurant, pk=pk)
          if form.is_valid():
              edit_review = form.save(commit=False)
              form.instance.posted_by = self.request.user
              print(edit_review)  # Print so I can see in cmd prompt that something posts as it should
              edit_review.save()
              return HttpResponseRedirect(reverse_lazy('restaurants:details', args=[restaurant.id]))
          return render(request, 'restaurants/details.html', {'form': form})
  </code></pre>
  
  <p>Do you have any idea of what might be going wrong?
  Thanks :)</p>