code: def save_model(self, request, entry, form, change):
          
          context = RequestContext(request)
          try:
              content = render_placeholder(entry.content_placeholder, context)
              entry.content = content or ''
          except KeyError:
              # https://github.com/django-blog-zinnia/cmsplugin-zinnia/pull/61
              entry.content = ''
          super(EntryPlaceholderAdmin, self).save_model(
              request, entry, form, change)
docs: Fill the content field with the interpretation
          of the placeholder