code: func (m *Manager) SaveAvatar(username string, avatar Avatar) error {
  	if err := m.RemoveAvatar(username, avatar.Label); err != nil {
  		return err
  	}
  	return m.getUserCollection().Update(
  		bson.M{"username": username},
  		bson.M{"$push": bson.M{"avatars": avatar}})
  }
docs: // SaveAvatar saves a new or updates an existing avatar