Bug 6977 : Adds support for repeatable subfields when importing authorities.

Bug 6977 : Adds support for repeatable subfields when importing authorities.

Before this patch, if we tried to import an authority with multiple $x
subfields into a bibliographic record, only the last value get added
to the form.

All repeated values should now be sent to the form.

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Ian Walls <ian.walls@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>