Merge: JSON fix

As @xymus reported in #1796, this PR should fix the aforementioned issue by closing an old TODO in `RopeBuffer::add` that was only supporting ASCII services.

At the same time, some cleanup was done in JSON to use properly our new Unicode-oriented strings that were worked around until now.

Closes #1796

Pull-Request: #1797
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Jean Privat <jean@pryen.org>