text: Title: MultiSelectField returns null during query
  Body: Hi 
  
  I am using the django-multiselectfield package and when I am making a query it always returns null. Am I missing some configuration because the other fields seems to working fine.
  
  My enum for the Multiselect field
  ```
  DAYS = (
      ('0', 'Sunday'),
      ('1', 'Monday'),
      ('2', 'Tuesday'),
      ('3', 'Wednesday'),
      ('4', 'Thursday'),
      ('5', 'Friday'),
      ('6', 'Saturday'))
  ```
  
  My model
  ```
  from multiselectfield import MultiSelectField
  
  class Queue(models.Model):
      days = MultiSelectField(choices=DAYS)
  ```
  
  My schema
  ```
  class QueueListType(DjangoListObjectType):
      class Meta:
          model = Queue
          pagination = LimitOffsetGraphqlPagination(default_limit=25)
  
  class Query(object):
      all_queues = DjangoListObjectField(QueueListType)
  ```
  
  When I am doing the query
  
  ```
  query {
    allQueues{
      totalCount
      results{
        id
        days
      }
    }
  }
  ```
  
  The result is 
  ```
  {
    "data": {
      "allQueues": {
        "totalCount": 2,
        "results": [
          {
            "id": "1",
            "days": null
          },
          {
            "id": "2",
            "days": null
          }
        ]
      }
    }
  }
  ```