callback function is not called as should

Server is a digest auth node.

```
HTTPDigest.prototype.request = function (data, options, callback) {
var self = this;
http.request(options, function (res) {
self._handleResponse(data, options, res, callback);
}).end();
};
```

if I do this in node I receive a 401 with nonce in order than to make the second request (the real one)
when browserified the a popup for the credentials is poped and the the function is not called but only after you click ok or cancel button.

首先，在代码经过browserify时，沿着所示的请求流程跟踪HTTPDigest.prototype.request，包括http.request和_handleResponse。在浏览器中重现401摘要认证响应，并比较凭据弹窗前后的回调行为。完成标准是：无需额外执行凭据对话框操作，回调即可一致地被调用。

由索引模型根据 Issue 内容生成。