model_to_dict not working with generic foreign keys
The `model_to_dict` accepts a parameter `extra_attrs` which you can use to do the lookup on the GFK object.
I am trying to include tags in object dict to convert this data to json then use this data in api how can i achieve that?(if models_to_dict can include tags in dict)
select query: 

`queryset = model.select()`

Tag Model:
```
class Tag(BaseDateTime):
    tag = CharField()
    object_type = CharField(null=True)
    object_id = IntegerField(null=True)
    object = GFKField('object_type', 'object_id')
```
model_to_dict
```
result = []
                for obj in queryset:
                    print obj.tags
                    result.append(model_to_dict(
                        obj, recurse=True, backrefs=True))
```
note: model create dynamically using : 
```
    def generate_XXXX_model(self):
        model_fields = {
            'text': CharField(index=True),
            'tags': ReverseGFK(Tag, 'object_type', 'object_id')
        }
        model = type(self.model_name, (BaseModel,), model_fields)
        model._meta.db_table = "XXXX_%s_XXXX" % self.table_name
        if not model.table_exists():
            model.create_table()
        _XXXX_XXX_models_cache[self.table_name] = model
        return model
```
result sample:
```
[
    {
        "username": "user3",
        "created": "2017-08-16T04:12:24.374450",
        "text": "api",
        "modified": "2017-08-16T04:12:24.374481",
        "feature_id": "11",
        "id": 2
    },
    {
        "username": "user2",
        "created": "2017-08-16T04:14:17.899443",
        "text": "api",
        "modified": "2017-08-16T04:14:17.899524",
        "feature_id": "13",
        "id": 3
    },
    {
        "username": "user1",
        "created": "2017-08-16T04:14:50.413285",
        "text": "api",
        "modified": "2017-08-16T04:14:50.413322",
        "feature_id": "16",
        "id": 4
    }
]
```