When a setting is set back to its default value, the value persisted in DB is `nil` to denominate that the default value should be used. However, the setting registry only looks at settings with non-nil values so it never picks up the nondefault->default transition.

Steps to reproduce:
1) Run rails console
2) Pick a $setting without custom value
3) Read a value of $setting with Setting[$setting]
4) In another process, change the value of $setting to a non-default one
5) Reload the settings with Foreman.settings.load_values
6) Repeat 3, notice the value is what was set in 4
6) In another process, change the value of $setting to the default one
7) Repeat 5
8) Repeat 3

Actual results:
The value in 8 is the same as in 6

Expected results:
The value in 8 is the same as in 3