How I can return json object in api controller?

Question

im using WebApi to make a webservice

as follow:

using RestSharp;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web.Http;
using Newtonsoft.Json;

namespace test.Controllers
{
    public class ValuesController : ApiController
    {
        public class WebAgent
        {
            public string Agent_no { get; set; }
            public string Agent_Name { get; set; }
        }

        public string GetAgent([FromBody] string a_body)
        {
            dynamic stuff = JsonConvert.DeserializeObject(a_body);
            string agent = stuff.agent_no.ToString();
            string name = stuff.agent_name.ToString();
            try
            {
                var test = new WebAgent()
                {
                    Agent_no = agent,
                    Agent_Name = name
                };
                JsonSerializerSettings settings = new JsonSerializerSettings
                {
                    NullValueHandling = NullValueHandling.Ignore,
                    Formatting = Formatting.Indented
                };
                var result= JsonConvert.SerializeObject(test,settings);
                return result;

            }
            catch (WebException e)
            {
                var message = e.Message;
                using (var reader = new StreamReader(e.Response.GetResponseStream()))
                {
                    var content = reader.ReadToEnd();
                    return content.ToString();
                }
            }
            finally
            {

            }
        }
    }
}

and everything just working fine

but when i try to call my webapi service from postman i got the following response

"{\r\n \"Agent_no\": \"122331\",\r\n \"Agent_Name\": \"sabatini\"\r\n}"

instead of: 

"{"Agent_no": "122331","Agent_Name": "sabatini"}"

i have tried to add

config.Formatters.JsonFormatter.S‌​erializerSettings = new JsonSerializerSettings
            {
                NullValueHandling = NullValueHandling.Ignore,
                Formatting = Formatting.Indented
            };

 to the WebApiConfig.cs but still get the same response !

i need help !

thanks