check for the existence of async.orig_callback since our callback may ex...

Hello @simulacre,

I love this project, but I ran into some problems while using it with sinatra-synchrony. It seems that the environment key gets changed to async.orig_callback when using sinatra-synchrony, so I added this hack to get it working. I'd like it to be a bit cleaner, but this is the best I could come up with at the time.

Thanks,
Allan