BUG Config::merge_array_low_into_high() ignores falsey values

Specific case: LeftAndMain::$session_keepalive_ping = true cannot be
set to false in config.yml for some cases because the value is ignored
when merge_array_low_into_high() is processing the config arrays.
Merge pull request #3272 from halkyon/config_fix

BUG Config::merge_array_low_into_high() ignores falsey values