code: func (s Store) GetValue(key string, v ...string) (string, error) {
  	kv, err := s.Get(key)
  	if err != nil {
  		if len(v) > 0 {
  			// Take default
  			return v[0], nil
  		}
  		return "", err
  	}
  	return kv.Value, nil
  }
docs: // GetValue gets the value associated with key. If there are no values
  // associated with key, GetValue returns "", ErrNotExist.