title: ManyRelatedManager object at [XXX] is not JSON serializable
body: <p>I have a simple model with <code>ManyToManyFields</code> I am serializing to JSON:</p>
  
  <p>models.py:</p>
  
  <pre><code>class Meeting(models.Model):
      site = models.ForeignKey(Site)
      meeting_title = models.CharField(default='', max_length=128, blank=True, null=True)
      meeting_team_members = models.ManyToManyField(Team, blank=False, null=False) # hosts
      meeting_visitors = models.ManyToManyField(Visitor, blank=False, null=False) # invited guests
  </code></pre>
  
  <p>views.py:</p>
  
  <pre><code>instances = Meeting.objects.filter(site=site.id)
  
  for ins in instances: #populate list
       context.append({
          'meeting_title': ins.meeting_title, 
          'meeting_team_members':ins.meeting_team_members.all, 
          'meeting_visitors': ins.meeting_visitors.all, 
  
  json_context = json.dumps(context, cls=DjangoJSONEncoder) #dump list as JSON
  return HttpResponse(json_context, 'application/javascript')
  </code></pre>
  
  <p>However I get the following TypeError:</p>
  
  <pre><code>TypeError at /dashboard/get_meetings/
  &lt;django.db.models.fields.related.create_many_related_manager.&lt;locals&gt;.ManyRelatedManager object at 0x03406730&gt; is not JSON serializable
  </code></pre>