code: func (m *Manager) SaveEmail(username string, email EmailAddress) error {
  	if err := m.RemoveEmail(username, email.Label); err != nil {
  		return err
  	}
  	return m.getUserCollection().Update(
  		bson.M{"username": username},
  		bson.M{"$push": bson.M{"emailaddresses": email}})
  }
docs: // SaveEmail save or update email along with its label