SPIGOT-6949: Configuration sections that are nested within Maps or Lists are not properly serialized.
This broke with the configuration changes in
3e2dd2bc120754ea4db193e878050d0eb31a6894.
This commit reverts one of the changes of this other commit so that
nested configuration sections are serialized as Maps again.
Although the types of these nested configuration sections are not
preserved when reloading a configuration (they turn into Maps), their
contents should at least be preserved, as it has been the case in
earlier Bukkit versions.
