code: func (t *Token) Extra(key string) interface{} {
  	if vals, ok := t.raw.(url.Values); ok {
  		// TODO(jbd): Cast numeric values to int64 or float64.
  		return vals.Get(key)
  	}
  	if raw, ok := t.raw.(map[string]interface{}); ok {
  		return raw[key]
  	}
  	return nil
  }
docs: // Extra returns an extra field.
  // Extra fields are key-value pairs returned by the server as a
  // part of the token retrieval response.