Bug 17913: Use AuthorityMergeMode pref in sub merge

Bug 17913: Use AuthorityMergeMode pref in sub merge

Original fix from a patch on bug 11315.
Amended by Marcel de Rooy January 2017.

Test plan:
If you set mode to loose, the test will still pass.
If you set mode to strict, one test will fail. (Fixed later.)

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>