code: func Filter(key, value string) RequestOptions {
  	return func(req *http.Request) {
  		q := req.URL.Query()
  		q.Add(key, value)
  		req.URL.RawQuery = strings.Replace(q.Encode(), "%2B", "+", 1)
  	}
  }
docs: // Filter applies a filter with the defined key and value.