text: Title: Incorrect model schema generation
  Body: Hi, I made a dynamic model serializer `UserCouponSerializer`, the fields of which vary according to `fields` argument passed when it initialized. If `fields` is None, it includes all fields of its model. The code is below.
  
  ```
  class DynamicFieldsModelSerializer(serializers.ModelSerializer):
      def __init__(self, *args, **kwargs):
          fields = kwargs.pop('fields', None)
          super(DynamicFieldsModelSerializer, self).__init__(*args, **kwargs)
          if fields is not None:
              allowed = set(fields)
              existing = set(self.fields)
              for field_name in existing - allowed:
                  self.fields.pop(field_name)
  ```
  
  ```
  class UserCouponSerializer(DynamicFieldsModelSerializer):
      id = serializers.CharField(max_length=20, read_only=True)
  
      class Meta:
          model = UserCoupon
          fields = '__all__'
  ```
  
  After that, I made another serializer `UserCouponListSerializer`, which uses `UserCouponSerializer` for its own fields(active_coupon_list, inactive_coupon_list). That two fields only needs partial fields from `UserCouponSerializer`, and I specified `fields` argument as seen below.  
  
  ```
  class UserCouponListSerializer(serializers.Serializer):
      active_coupon_list = serializers.ListField(
          child=UserCouponSerializer(
              fields=['affiliate', 'goods_name', 'period_end', 'status', 'id', 'pay_id']
          )
      )
      inactive_coupon_list = serializers.ListField(
          child=UserCouponSerializer(
              fields=['affiliate', 'goods_name', 'period_end', 'status', 'id', 'pay_id']
          )
      )
  
  ```
  
  However, the problem is, when I rendered API documentation, everything generated from  `UserCouponSerializer` includes only the fields specified in `UserCouponListSerializer`, that is, 'affiliate', 'goods_name', 'period_end', 'status', 'id', and 'pay_id'. 
  
  ![image](https://user-images.githubusercontent.com/24687378/66375448-5f2d4e80-e9e8-11e9-8b4b-cde401fa9e16.png)
  
  I expected all fields of `UserCoupon` fields are rendered as I defined `UserCouponSerializer`.
  
  Can I know the cause and get some help about that?
  
  