Fixed async callback to handle error objects

I was trying out supertest, which is an excellent library for testing express applications (https://github.com/visionmedia/supertest). It states that it supports all frameworks, but I ran into a problem with Buster. It seems to be a general convention that the "done" async callback can receive an Error object as its first argument, but Buster does not handle that.

So added the line below. Hopefully it is a good place to put it. I could not find any tests related to this functionality so I did not write one.