code: func (c *Client) SignHandler() (string, error) {
  	if err := c.condDiscovery(); err != nil {
  		return "", err
  	}
  	if c.signHandler == "" {
  		return "", ErrNoSigning
  	}
  	return c.signHandler, nil
  }
docs: // SignHandler returns the server's sign handler.
  // If the server isn't running a sign handler, the error will be
  // ErrNoSigning.