Bug 20971: Prevent Storable::thaw to fail on LastCreatedItem

Bug 20971: Prevent Storable::thaw to fail on LastCreatedItem

Sometimes additem.pl will complain about "Storable::thaw failed to
thaw LastCreatedItem-cookie.", see bug 14844. Now, actually fix the bug.

The bug is caused by trying to URI (un)escape MARC::Record, binary data.
We'll use a base64 url-safe version instead.

Test plan:
1: Set PrefillItem to 'The new item is prefilled...'
2: Set a SubfieldsToUseWhenPrefill, 'c' for example
3: Add a new item for biblio A with 'c' set.
4: Double check 'c' value is set for next new item A.
5: search and add a new item for biblio B
6: 'c' is not set
Apply patch
7: logout, login
8: repeat steps 3, 4, and 5 .
9: now 'c' is set.

Signed-off-by: Pasi Kallinen <pasi.kallinen@joensuu.fi>
Signed-off-by: James O'Keeffe <jamespfk@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
JD amended patch: fix commit title and add test plan
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>