[stable/beryllium] router-gateway-set with multiple fixedIPs resulting in only fixedIP per subnet getting written to MDSAL

Upon updating a router with router-gateway-set with multiple fixed IPs per subnet, only one is getting reflected in neutron router config DS.

Please see the attachment for detailed DS inputs/outputs and openstack commands to reproduce.

In AbstractNeutronNorthboundIAware, update is sending two IPs, but a GET on update is returning just one.