title: Error with list_route: POST methods in Viewset
body: <p>I am making a function Add Friend which user can Create one Friendship Request object to another user user URL:<code>http://localhost:8000/api/v1/users/{username}/add_friend/</code> but I get error when making.</p>
  
  <p><strong>Code in Urls.py</strong></p>
  
  <pre><code>from .views import (AddFriendUserAPIView)
  urlpatterns = [
      url(r'^(?P&lt;username&gt;[\w-]+)/add_friend/$', AddFriendUserAPIView.as_view({'post': 'add_friend'}), name='add_friend'),
  ]
  </code></pre>
  
  <p><strong>Code in Views.py</strong></p>
  
  <pre><code>class AddFriendUserAPIView(ReadOnlyModelViewSet):
      serializer_class = FriendRequestSerializer
      queryset = FriendshipRequest.objects.all()
      lookup_field = 'username'
  
      @detail_route(methods=['POST'])
      def add_friend(self, request, username):
          to_user = User.objects.get(username=request.data['to_user_username'])
          add_qs = FriendshipRequest.objects.get_or_create(from_user=request.user, to_user=to_user , message='')
          serializer = FriendRequestSerializer(add_qs)
          return Response(serializer.data)
  </code></pre>
  
  <p><strong>Code in serializers.py</strong></p>
  
  <pre><code>class FriendRequestSerializer(ModelSerializer):
      class Meta:
          model = FriendshipRequest
          fields = (
              'id',
              'from_user',
              'to_user',
              'message', 
          )
  </code></pre>
  
  <p>Code in <strong>Models</strong> of Friend Requests:</p>
  
  <pre><code>class FriendshipRequest(models.Model):
      from_user = models.ForeignKey(User, related_name='friendship_requests_sent')
      to_user = models.ForeignKey(User, related_name='friendship_requests_received')
      message = models.TextField(blank=True)
  </code></pre>
  
  <p><strong>Error log by Trackback</strong></p>
  
  <pre><code>    Traceback:
  
  File "C:\Python27\lib\site-packages\django\core\handlers\exception.py" in inner
    41.             response = get_response(request)
  
  File "C:\Python27\lib\site-packages\django\core\handlers\base.py" in _get_response
    187.                 response = self.process_exception_by_middleware(e, request)
  
  File "C:\Python27\lib\site-packages\django\core\handlers\base.py" in _get_response
    185.                 response = wrapped_callback(request, *callback_args, **callback_kwargs)
  
  File "C:\Python27\lib\site-packages\django\views\decorators\csrf.py" in wrapped_view
    58.         return view_func(*args, **kwargs)
  
  File "C:\Python27\lib\site-packages\rest_framework\viewsets.py" in view
    90.             return self.dispatch(request, *args, **kwargs)
  
  File "C:\Python27\lib\site-packages\rest_framework\views.py" in dispatch
    489.             response = self.handle_exception(exc)
  
  File "C:\Python27\lib\site-packages\rest_framework\views.py" in handle_exception
    449.             self.raise_uncaught_exception(exc)
  
  File "C:\Python27\lib\site-packages\rest_framework\views.py" in dispatch
    486.             response = handler(request, *args, **kwargs)
  
  File "C:\Users\User\Desktop\FeedGit\backend\api\authentication\add_friend.py" in add_friend
    16.         to_user = User.objects.get(username=request.data['to_user_username'])
  
  File "C:\Python27\lib\site-packages\django\utils\datastructures.py" in __getitem__
    85.             raise MultiValueDictKeyError(repr(key))
  
  Exception Type: MultiValueDictKeyError at /api/v1/users/duongnuhabang/add_friend/
  Exception Value: "'to_user_username'"
  </code></pre>