NSMutableURLRequest在json对象

Question

我在http post请求的正文中发送以下JSON

{"username":"admin", "password":"admin"}

这是代码

let jsonData = try NSJSONSerialization.dataWithJSONObject(SerializationHelper.toDictionary(user), options: .PrettyPrinted
        )
        let request = NSMutableURLRequest(URL: NSURL(string: baseUrl + "/users/login/")!)

        request.HTTPMethod = "POST"


        request.HTTPBody = jsonData

        let task = NSURLSession.sharedSession().dataTaskWithRequest(request) { data, response, error in
            guard error == nil && data != nil else {                                                          // check for fundamental networking error
                print("error=\(error)")
                return
            }

            if let httpStatus = response as? NSHTTPURLResponse where httpStatus.statusCode != 200 {           // check for http errors
                print("statusCode should be 200, but is \(httpStatus.statusCode)")
                print("response = \(response)")
            }

            let responseString = String(data: data!, encoding: NSUTF8StringEncoding)
            print("responseString = \(responseString)")
        }
        task.resume()


    } catch {
        print("error")
        print(error)
    }

当在服务器上接收到正文时，它是

{ '{"password":"admin","username":"Admin"}': '' }

为什么将身体设置为另一个json对象的键？这是默认行为还是我做错了什么？