Android开发，仿写开源中国客户端?报错

听马伟奇的《十分钟完成开源中国》仿写，写登陆模块时访问https://www.oschina.net/action/api/login_validate

提交参数"keep_login","username","pwd"无响应，不能返回登陆结果字符串，请问这是怎么回事？

public class LoginActivity extends AppCompatActivity {

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);
        Request request = ItheimaHttp.newGetRequest("action/api/login_validate");
        request.putParams("keep_login",1)
                .putParams("username","xx@qq.com")
                .putParams("pwd","xx");
        Call call = ItheimaHttp.send(request, new HttpResponseListener<String>() {

            @Override
            public void onResponse(String s, Headers headers) {
                System.out.println(s);
            }
        });
    }
}

响应函数是运行了的，但是字符串里面是空的，日志也没有报错。