Bug 19537: Correct addition of source subfield from authorities

Bug 19537: Correct addition of source subfield from authorities

To test:
1 - Find or create a subject authority
2 - Define 008/11 as 's' or 'r'
3 - Add 2nd indicator '7' to main heading
4 - Open or create a bibliographic record
5 - Link to the authority created above
6 - Subfield $2 is not set
7 - Apply patch
8 - Repeat linking, subfield $2 should be set

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>