Bug 15429 UT for _parseletter modifying its parameter

Bug 15429 UT for _parseletter modifying its parameter

Before/after applying the related patch, do:

   prove -v t/db_dependent/Letters.t

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit fd07e3296b64a78edf1cf947a824147f1adbba1a)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
(cherry picked from commit 2eac97ba65d869f34641f6eb7fd22047a104a794)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>