Bug 13360: C4::Ris assumes that hash keys are ordered - KW

Bug 13360: C4::Ris assumes that hash keys are ordered - KW

This patch only fixes the KW order.

Test plan:
1/ Choose/create a record with several 6XX (for KW), see the code source
to know which fields you can use
2/ Export this record in RIS format
3/ Verify that the KW lines are ordered following the marc record fields
order.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
We really should refactor this whole thing into Koha::RIS sometime, it's
a horrible module at the moment.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>