code: func (m *GorillaHost) Build(u *url.URL, values url.Values) error {
  	host, err := m.RevertValid(values)
  	if err == nil {
  		if u.Scheme == "" {
  			u.Scheme = "http"
  		}
  		u.Host = host
  	}
  	return err
  }
docs: // Build builds the URL host using the given positional and named variables,
  // and writes it to the given URL.