code: func (token *JSONWebToken) decodeHeader() (err error) {
  	var h ast.Value
  	if h, err = builtinBase64UrlDecode(ast.String(token.header)); err != nil {
  		return fmt.Errorf("JWT header had invalid encoding: %v", err)
  	}
  	if token.decodedHeader, err = validateJWTHeader(string(h.(ast.String))); err != nil {
  		return err
  	}
  	return
  }
docs: // decodeHeader populates the decodedHeader field.