text: Title: ObjectType resolving fields from dicts or objects
  Body: Hi! There is an example code:
  
   
  ```
  class Photo(graphene.ObjectTypet):
    
        id = graphene.ID()
        url = graphene.String()
        fileName = graphene.String()
        extension = graphene.String()
    
    
  class User(graphene.ObjectTypet):
    
        _id = graphene.ID()
        name = graphene.String()
        photos = graphene.List(Photo)
  
  class Query(graphene.ObjectType):
        user = graphene.Field(User, dict(_id=graphene.String(required=True)))
    
        async def resolve_user(self, info, _id):
            user = await db.users.find_one(dict(_id=_id))
            # do the simple filted
            resolved_fileds = {f: user[f]
                             for f in User._meta.fields}
            user = User(**resolved_fileds) if user else None
            return user
  ```
  The result is
  
  ```
  {
    "data": {
      "user": {
        "Id": "1",
        "name": "Test User",
        "photos": [
          {
            "id": null,
            "fileName": null
          }        
        ]
      }
    }
  }
  ```
  
  Field "photos" not initialized in __init__. In argument "photos" I got just list with dicts. I know  that I can to this serialization by myself, but maybe it's already implemented?