Fix delayed queue now behaviour

Hi, I found "Queue now" on Delayed server panel to never queue a future job for me.

A way to make it work, was to not call `Resque::Scheduler.enqueue_delayed_items_for_timestamp` and bypass the is_master? check. Seems the master lock is never acquired when calling `Resque::Scheduler::Server.delayed_queue_now`.

I might doing the wrong thing here, but I would like to get some feedback on this if possible.
Why queue now might not work for me? Are there some corner cases I am missing here?
This will only enqueue the *first* item at the given timestamp.