[5.x] Fix delayed until on pending jobs screen

[paras-malhotra]

Fixes #906

I've confirmed this is a bug in 5.x as well. The issue is that PendingDispatch::delay accepts an int or DateTime. The current Horizon code assumes that it's always a date. This PR fixes that. I've checked it on my PC.

Steps to reproduce:

1. Execute TestJob::dispatch(...)->delay(60*60*24);
2. Go to the pending jobs screen. On the index screen, it shows "delayed for a few seconds" (as tooltip) and on the job detail screen, it shows Delayed Until as the current time.

[stefanbauer]

Thanks for fixing it @paras-malhotra 👍