code: func tokenHeaderString(name string, where *string, value ast.Value) (err error) {
  	var ok bool
  	var v ast.String
  	if v, ok = value.(ast.String); !ok {
  		err = fmt.Errorf("%s: must be a string", name)
  		return
  	}
  	*where = string(v)
  	return
  }
docs: // tokenHeaderString handles string-format JWT header parameters