title: success_message in DeleteView not shown
body: <p>I have a DeleteView: </p>
  
  <pre><code>class LectureDelete(SuccessMessageMixin, DeleteView):
  model = Lecture
  success_message = "Die Veranstaltung wurde gelöscht"
  success_url = '/'
  
  def get_object(self):
      qs = super(LectureDelete, self).get_object()
      if self.request.user.has_perm('edit_lecture', qs):
          return qs
      else:
          raise exceptions.PermissionDenied
  </code></pre>
  
  <p>And in my template to which the success_url links, I have the following code, which works fine with other messages: </p>
  
  <pre><code>{% if messages %}
          {% for message in messages %}
          &lt;p class="alert alert-dismissable {% if message.tags %}alert-{{ message.tags }}"{% endif %}&gt;
              &lt;button type="button" class="close" data-dismiss="alert" aria-hidden="true"&gt;&amp;times;&lt;/button&gt;
              {{ message }}
          &lt;/p&gt;
          {% endfor %}
      {% endif %}
  </code></pre>
  
  <p>But the message is not shown. Am I missing something? What am I doing worng? Thanks!</p>