Keys with explicit indexes (e.g. foo[bar]) were handled only
partially before this change and in quite a crude way. The main
new feature (or fixed bug if you prefer) is that map keys with
periods were not identified correctly, so foo[bar.spam] came out
wrong. By scanning the key for indexes "[]" first, we can split
the logic better to make sure that all keys can be explicitly
indexed if desired. I believe this would work in YAML as well
but it definitely works for properties file inputs.

Fixes gh-2387 (I believe)