"Callback was already called" error

[vjaitly]

Post the 0.9.3 build q.del() and q.msg_reserve() methods are failing with
"callback was already called" error.

The source code shows the following line ( which was added for making callback optional)
return typeof cb === "function" ? cb(cb(error, body)) : void 0;

This seems to be causing the issue as the callback is being called twice.