Callback undefined on subsequent poll() calls

If the worker is running in non-looped mode (for testing in my case) the
callback is undefined on the first `poll()` recursion.  This means that
if a job is found (or all queues have been polled) the callback
originally passed to poll is lost.
Merge pull request #55 from cheesefactory/fix-non-looping-poll

Callback undefined on subsequent poll() calls