You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: exchange_refresh_token.go
    path of file: ./repos/dotenx/ao-api/oauth
    
    the code of the file until where you have to start completion: package oauth

import (
	"bytes"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"log"
	"net/http"
	"net/url"
	"time"

	"github.com/dotenx/dotenx/ao-api/models"
	"github.com/dotenx/dotenx/ao-api/pkg/utils"
	"github.com/dotenx
    
func gumroadRefreshToken(clientId, clientSecret, refreshToken string) (accessToken, newRefreshToken string, err error) {
	var dto struct {
		AccessToken  string `json:"access_token"`
		RefreshToken string `json:"refresh_token"`
	}
	data := "refresh_token=" + refreshToken
	data += "&grant_type=refresh_token"
	data += "&client_id=" + clientId
	data += "&client_secret=" + clientSecret
	url := "https://api.gumroad.com/oauth/token"
	headers := []utils.Header{
		{
			Key:   "Content-Type",
			Value: "application/x-www-form-urlencoded",
		},
	}
	body := bytes.NewBuffer([]byte(data))
	helper := utils.NewHttpHelper(utils.NewHttpClient())
	out, err, status, _ := helper.HttpRequest(http.MethodPost, url, body, headers, time.Minute, true)
	logrus.Info("gumroad response:", string(out))
	if err != nil {
		return "", "", err
	}
	if status != http.StatusOK && status != http.StatusCreated {
		return "", "", errors.New("not ok with status " + fmt.Sprint(status))
	}
	err = json.Unmarshal(out, &dto)
	return dto.AccessToken, dto.RefreshToken, err
}