Compatibility with superagent-bluebird-promise

[zachlysobey]

I'm using your library in conjunction with superagent-bluebird-promise, but it seems not to work for non-200 responses.

The following superagent-bluebird-promise code ends up causing an error:

Uncaught TypeError: Cannot read property 'status' of null

if (typeof res !== "undefined" && res.status >= 400) {
  var msg = 'cannot ' + req.method + ' ' + req.url + ' (' + res.status + ')'

From: https://github.com/KyleAMathews/superagent-bluebird-promise/blob/master/index.js#L66-L67

This is happening when superagent-mocker calls that function with res passed in as null rather than undefined.

if (response.status !== 200) {
  cb && cb(response, null);

From: https://github.com/A/superagent-mocker/blob/master/index.js#L99-L100

I'm not entirely sure which of your libraries is responsible for this bug, but either case looks like a pretty easy fix. (I've submitted an issue to superagent-mocker as well: KyleAMathews/superagent-bluebird-promise#53)

If you want, I can submit a PR to change

cb && cb(response, null);

to

cb && cb(response, undefined);

or

cb && cb(response);

Either change could be a quick fix for my issue.

Thanks for taking your time to contribute this very useful library to the OSS community!