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.

Perhaps this falsey check should instead be moved into the `isset($dest[$k]) {}` block? It is ok to merge falsey values that DON'T have an existing value in the destination.