plus.net.XMLHttpRequest（）post 传送json对象失败

最后请求失败，正确的post怎么写，传json

var xhr = null;  
function testXHR() {  
    if (xhr) {  
        return;  
    }  
    xhr = new plus.net.XMLHttpRequest();  
    xhr.onreadystatechange = xhrStatechange;  

    //  xhr.open("GET", "http://www.yohaschool.com/dan/dan.ashx?dataA=11");  
    var data = {  
        dataA: "11"  
    };  
    xhr.open("POST", "http://www.yohaschool.com/dan/dan.ashx");  

    xhr.responseType = "json";  
//                                  xhr.setRequestHeader("dataType", "text");  
    //  xhr.setRequestHeader("Content-Type", "text/json");  

    //xhr.send(JSON.stringify(data));  
//                                  alert(JSON.stringify(data));  
    xhr.send(data);  
}  

function xhrStatechange() {  

    switch (xhr.readyState) {  
        case 0:  
            console.log("xhr请求已初始化");  
            break;  
        case 1:  
            console.log("xhr请求已打开");  
            break;  
        case 2:  
            console.log("xhr请求已发送");  
            break;  
        case 3:  
            console.log("xhr请求已响应");  
            break;  
        case 4:  
            if (xhr.status == 200) {  
                alert("xhr请求成功：" + xhr.responseText);  

            } else {  
                console.log("xhr请求失败：" + xhr.readyState);  
            }  
            break;  
        default:  
            break;  
    }  
}