Fix code duplication for merge/personalization feature

[racke]

Same block of code appears four times in `Sympa::Spindle::AuthorizeMessage which is a violation of the DRY rule.

        # Check syntax for merge_feature.
        if (    $message->{shelved}{merge}
            and $message->{shelved}{merge} ne 'footer'
            and not _test_personalize($message, $list)) {
            $log->syslog(
                'err',
                'Failed to personalize. Message %s for list %s was rejected',
                $message,
                $list
            );
            Sympa::send_dsn($list, $message, {}, '5.6.5');
            return undef;
        }