[Koha] Fix confusion between 099 and 942 field in UNIMARC framework

Hi Some weeks ago I submitted a patch in order to fix the management of Koha-specific fields in UNIMARC. https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=38336 If you're familiar with the way Koha manage UNIMARC and think you can make a constructive comment, don't hesitate. Even if don't know UNIMARC you should be able to test easily the patch and sign if off by following the test plan. Regards, Mathieu Saby, Université Cote d'Azur