fix: set_config

[Danielkonge]

This code fixes the problem I mention with ibl.overwrite() in #778. With this bug fix I get the expected behavior mentioned in that issue. (This also closes that issue, since I get the behavior I was asking for.)

[lukas-reineke]

This is wrong, setup and by extension set_config explicitly resets to the default. From the help

setup is idempotent, meaning you can call it multiple times, and each call will reset indent-blankline. If you want to only update the current configuration, use |ibl.update()|.

Also, ibl.overwrite does not call set_config, I'm not sure I understand what your problem is.