Bug 22667: Pick default value from framework for cn_source

Bug 22667: Pick default value from framework for cn_source

If the cn_source has a default value defined in the framework it will
not be used when ordering or receiving serials.

Test plan:
Set the pref DefaultClassificationSource to Library of congress
Define a default value=ddc (for Dewey) for the ACQ and default framework
Receive an order (AcqCreateItem=on ordering) and confirm that Dewey is
selected by default (the default value from the ACQ framework is used)
Receive a serial and confirm that Dewey is selected by default

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>