[5.2] Forget key when key exists

[marcvdm]

When using for example ip addresses as keys for an array and try to use array_forget() or Arr::forget() it wil not check if the key exists in the array before exploding the key.

This wil, like Arr::get() or Arr::has(), first check if the key exists.

[GrahamCampbell]

Could you add tests please?