Bug 37183: Batch edit serial subscriptions sets expiration date to today

Bug 37183: Batch edit serial subscriptions sets expiration date to today

Test plan:

Add some serials:
1) Add a new serial, visit:
   /cgi-bin/koha/serials/subscription-add.pl
2) Put a biblionumber in the 'record' field, e.g. '112'.
   Press 'next' and click 'ok' on the alert box.
3) Fill all the required fields and click 'test prediction'.
4) Fill the Subscription end date (= Expiration Date).
5) Click 'save subscription'.
6) Repeat steps 1-5 to create a second serial.

Batch edit serials:
1) Visit serials and hit the 'Search' button:
   /cgi-bin/koha/serials/serials-home.pl
2) Click the 2 checkboxes for the 2 serials we created
   previously and click the new link that pops up
   'Edit selected serials'.
3) Click 'Save' without changing anything.
4) Go back to either of the serials, notice the value
   for Expiration date is changed to TODAY
   (the date of the batch edit).

Apply the patch and retest the batch editing (before
retesting, change the expiration dates of the two
serials back to the original expiration date).

Note that the expiration date now only changes if you
enter a date in the 'Expiration date' field.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>