Bug 35725: Keep cardnumber and library data when changing request type

Bug 35725: Keep cardnumber and library data when changing request type

Test plan:
1) Apply 35570
2) Enable ILLModule sys pref through UI, or run:
   echo "update systempreferences set value = 1 where variable = \"ILLModule\";" | koha-mysql kohadev
3) Create a new Standard request, visit:
   http://localhost:8081/cgi-bin/koha/ill/ill-requests.pl?method=create&backend=Standard
4) Input a cardnumber and/or set a library at the bottom under "Patron options".
5) Change the type (top of the form). Notice the data from step 4) is gone.
6) Apply patch. Repeat test plan and notice data is kept when changing request type.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>