title: Return only funded fields in response(Django Rest SearchFilter)
body: <p>I have <code>ViewSet</code> which have <code>SearchFilter</code> in <code>filter_backends</code> and two search_fields. How to return JSON represents the only field results?
  For example if i send response for search with "jo" substring i will have only one field for this search(funded user name or full name), not both user name and email in each JSON object:</p>
  
  <pre><code>{
      [{
          "username": "jonh";
       },
       {
          "fullname": "Jonh";
       },
       {
          "username": "jo";
       },
       {
          "fullname": "Johnson";
       }
      ]
  }
  </code></pre>
  
  <p>viewsets.py:</p>
  
  <pre><code>class UserViewSet(mixins.RetrieveModelMixin,
                    mixins.UpdateModelMixin,
                    mixins.ListModelMixin,
                    viewsets.GenericViewSet):
  
      queryset = User.objects.all()
      permission_classes = (IsUserOrReadOnly,)
      filter_backends = (filters.SearchFilter,)
      search_fields = ('^username', '^fullname',)
  
      def get_serializer_class(self):
          if hasattr(self, 'action') and self.action == 'list':
              return UserListSerializer
          return UserDetailSerializer
  </code></pre>
  
  <p>serializers.py:</p>
  
  <pre><code>class UserListSerializer(serializers.ModelSerializer):
  
      class Meta:
          model = User
          fields = ('id', 'username', 'email', 'fullname')
  </code></pre>