title: How to filter an object with choice filed values in django_filter
body: <p>I have following choice field in my model IPInfo</p>
  
  <pre><code>class IPInfoModel(models.Model):
      TYPE_INTRANET = 1
      TYPE_INTERNET = 2
      IP_TYPES = (
          (TYPE_INTRANET, u'INTRANET'),
          (TYPE_INTERNET, u'INTERNET'),
      )
      ip = models.GenericIPAddressField("IP", unique=True)
      ip_type = models.SmallIntegerField(choices=IP_TYPES)
  
  </code></pre>
  
  <p>and I use django_filters to filter IPInfo.</p>
  
  <pre><code>from django_filters import rest_framework as django_filters 
  
  class IPInfoFilter(django_filters.FilterSet):
      ip_type = django_filters.ChoiceFilter(choices=IPInfoModel.IP_TYPES)
  
      class Meta:
          model = IPInfoModel
          fields = ["ip_type",]
  
  
  class IPInfoViewSet(mixins.ListModelMixin, viewsets.GenericViewSet):
      queryset = IPInfoModel.objects.all()
      serializer_class = IPInfoSerializer
      filter_class = IPInfoFilter
  
  </code></pre>
  
  <p>I want to filter IPInfo on ip_type. How can I filter IPInfo by either "INTRANET" or "INTERNET". not use "1" or "2".</p>