Bug 15429 - sub _parseletter should not change referenced values

Bug 15429 - sub _parseletter should not change referenced values

In C4/Letters.pm, sub _parseletter changes values that are passed by
reference. E.g. patron's expiry date can change from iso format to
syspref format, leading to strange behaviour in the calling routines
(see Bug 15423).

This patch makes sub _parseletter work on a copy of the referenced values.

(Submitted to get feedback - is this the way to go?)

Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
  Good solution to real time bomb.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>