Bug 19503: Duplicating a dropdown menu subfield yields an empty subfield tag

Bug 19503: Duplicating a dropdown menu subfield yields an empty subfield tag

While editing a record in the staff client, if you clone a repeatable
dropdown subfield the cloned subfield's tag is empty. This can result
in data loss if the record is saved, re-opened, and saved yet again.
This patch (originally written by Sophie MEYNIEUX for bug 17818) fixes that.

Test plan:
0) [PREREQUISITE] In your MARC framework (Home > Administration > MARC
   bibliographic framework) ensure that you have at least one subfield
   of a particular tag linked to an authorised value (e.g. in UNIMARC,
   tag 700 subfield 4 is 'Relator Code' and can be linked to CCODE for
   testing purposes). This is so that the relevant subfield will be a
   dropdown menu and not a textbox.
1) In the Staff Client, edit an existing record or create a new one.
   Then, try to clone any subfield that is a dropdown menu. Observe
   that the cloned subfield's tag is empty.
2) Apply the patch.
3) Hit CTRL-F5 in your browser (to ensure cataloging.js is re-loaded)
   and try to clone a dropdown menu subfield again. This time the tag
   is cloned as well.

Working as intended.

Signed-off-by: Simon Pouchol <simon.pouchol@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
https://bugs.koha-community.org/show_bug.cgi?id=16503

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 4f2d59e67b6d938ca2798fe8b7fc3ec6eda1d253)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>