webapi返回不带引号的字符串，解决自动加双引号的问题

返回类型改为HttpResponseMessage类型

 [Route("api/TestControllers/test")]
 [HttpGet]
 public HttpResponseMessage Test()
 {
     HttpResponseMessage responseMessage = new HttpResponseMessage { Content = new StringContent("你要返回的字符串", Encoding.GetEncoding("UTF-8"), "text/plain") };

     return responseMessage;
 }