public async Task <IActionResult> PostUser([FromBody] User user)
        {
            if (await userService.CheckUsernameExists(user.UserName))
            {
                return(new StatusCodeResult(StatusCodes.Status409Conflict));
            }

            if (!ModelState.IsValid)
            {
                return(BadRequest(ModelState));
            }

            userService.CreateUser(user);
            await userService.SaveUser();

            return(CreatedAtAction("GetUser", new { id = user.UserName }, user));
        }