Add mock status code

[khmelevskii]

for example, mock status code = 404

[A]

Nice idea. I'll try to hack it.

[A]

I think, I can reuse some things from superagent.Response constructor to provide same API. But it's looks like the new major version 'cause it will break current API. But I think, that this is the right way. Give me some additional time please.

[LeopoldoFu]

I think I've got something workable (works for my tests): https://github.com/ebsco/superagent-mocker/tree/mockable-status-codes

[A]

@LeopoldoFu Super! Could you make pull request?

[lobobabysaurus]

@LeopoldoFu :+1: to a pull request to support this