Added failing test case when `Que.mode = :sync' and using specific `run_at'

Hey there!

I'm really liking Que thus far - and have been playing around with the ActiveJob integration, but I _think_ I might've come across a bug when `Que.mode = :sync`.

I noticed that when the `Que.connection = nil` and `Que.mode = :sync` and you attempt to enqueue a job with a specific `run_at`, Que with throw a runtime error with the message "Que connection not established!". I added a failing test case in this commit to demonstrate.

I wanted to patch it too - but i'm not 100% sure where to start digging, or if it was even a bug (and not just me :smile:). I stumbled across this whilst adding support for `enqueue_at` in ActiveJob.