原生AJAX post请求访问后端JSON数据接口

function testAvatar() {
    var data = {
        "username": "123123",
        "password": "123"
    }

    var xmlHttp = new XMLHttpRequest();
    xmlHttp.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
            console.log(this.responseText);
            var i = JSON.parse(this.responseText);
        }
    }
    xmlHttp.open('POST', 'http://localhost:8080/account/login', true);
    xmlHttp.setRequestHeader('Content-Type', 'application/json')
    xmlHttp.send(JSON.stringify(data))
}

如果没该Request请求头则会报415错误