code: func (m *Manager) SaveAddress(username string, address Address) error {
  	if err := m.RemoveAddress(username, address.Label); err != nil {
  		return err
  	}
  	return m.getUserCollection().Update(
  		bson.M{"username": username},
  		bson.M{"$push": bson.M{"addresses": address}})
  }
docs: // SaveAddress save or update address