code: func (c CookieOptions) getCookieValue(w http.ResponseWriter, r *http.Request) (string, error) {
  	cookie := w.(cookieWriter).GetCookie(c.Name)
  	if cookie != nil {
  		return cookie.Value, nil
  	}
  
  	reqCookie, err := r.Cookie(c.Name)
  	if err != nil {
  		return "", errNoSession{}
  	}
  
  	return reqCookie.Value, nil
  }
docs: // getCookieValue returns the cookie value (usually the ID of the session)
  // stored in the cookies cache. If it does not exist in the cookies cache
  // it will attempt to fetch it from the request headers.
  // If this fails it will return nil.