improve contact fixing of nat helper in requests, so that via port is used when rport is empty.
improve contact fixing of nat helper in requests, so that via port is used when rport is empty.

biddyweb/flexisip,gale320/flexisip,biddyweb/flexisip,BelledonneCommunications/flexisip,biddyweb/flexisip,gale320/flexisip,BelledonneCommunications/flexisip,biddyweb/flexisip,BelledonneCommunications/flexisip,gale320/flexisip,BelledonneCommunications/flexisip,gale320/flexisip