text: Title: bson.errors.InvalidDocument: cannot encode object: frozenset()/set()
  Body: Exception while saving an object containing either a frozenset or a set:
  ```py
  from odmantic import Model, AIOEngine
  from odmantic.bson import ObjectId
  
  class UserModel(Model):
      username: str
      favorites_article_ids: FrozenSet[ObjectId] = frozenset()
  
  engine = AIOEngine()
  await engine.save(UserModel(username="jean")) 
  ```