Fix GoodJob::Setting duplicate keys

[mohammednasser-32]

fix #1490

The problem was that key is received as a symbol, while the settings array store the value in the database as strings, so

["heartbeat_schedules"].includes?(:heartbeat_schedules)
false

so we need to convert the key to string first

I also refactored cron_key_enabled test to be consisten with cron_key_disabled test

[bensheldon]

@mohammednasser-32 thank you! I just had to fix CI on main. I'm going to let CI run through (I feel confident it will be green) and I'll merge this.

[mohammednasser-32]

@bensheldon Thank you! 🙏