nta.c: when sending ACK to [3456]XX, copy the To header as-is from the response
nta.c: when sending ACK to [3456]XX, copy the To header as-is from the response

Problem reported by Paulo Pizarro.

darcs-hash:20090115180600-db55f-d8465fb2460473a2e574f471eabba10c140b2b16.gz

BelledonneCommunications/sofia-sip,xhook/sofia-sip,xhook/sofia-sip,jart/sofia-sip,BelledonneCommunications/sofia-sip,erdincay/sofia-sip,erdincay/sofia-sip,xhook/sofia-sip,unispeech/sofia-sip,unispeech/sofia-sip,BelledonneCommunications/sofia-sip,BelledonneCommunications/sofia-sip,unispeech/sofia-sip,erdincay/sofia-sip,xhook/sofia-sip,erdincay/sofia-sip,unispeech/sofia-sip,jart/sofia-sip,jart/sofia-sip,xhook/sofia-sip