BUG If config key doesn't exist, false config vals aren't merged.

Config::merge_array_low_into_high() will ignore false values that
don't exist in the config already. The expectation is that if the
key isn't set, you can still set a false value, just not overwrite
the true value with a false one when merging into an existing.

Added some tests to verify that merging still works as expected.
I think that should be okay :)