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.

Start by tracing HTTPDigest.prototype.request in the shown request flow, including http.request and _handleResponse, when the code is browserified. Reproduce the 401 digest-auth response in the browser and compare callback behavior before and after the credentials popup. Done means the callback is invoked consistently without requiring an extra credentials-dialog action.

Written by the indexing model from the issue text.