Bug 20461: Serials: fix ignored params in subscription creation form

Bug 20461: Serials: fix ignored params in subscription creation form

In subscription-add.pl, the two params aren't passed to
NewSubscription() but they are to ModSubscription()

== Test plan ==
1. Enable syspref "makePreviousSerialAvailable"
2. Create a minimal subscription with a value in the "Item type" and
     "item type for older issues" fields.
3. Edit the subscription
4. You should see that the two fields are empty. This is the bug.
5. Apply this patch
6. Do step 2 and 3
7. You should see that the two fields have the right value
8. Express the joy of a successful and easy sign off.
   (this is important, otherwise the sign off spell won't work!)

Signed-off-by: delaye <stephane.delaye@biblibre.com>
Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>