Changes contact use in build_peer to use the FORCE_RPORT flag instead of RPORT_PRESENT

Changes contact use in build_peer to use the FORCE_RPORT flag instead of RPORT_PRESENT

It turned out that this was causing NAT=Yes to always use rport when present which was
against 1.6.2 behavior and the check itself was redundant since the only way this
segment of code could be reached was if RPORT_PRESENT was already evaluated as true
earlier.

(closes issue ASTERISK-17789)
Reported by: byronclark
Patches:
      use_sip_nat_force_rport.patch uploaded by byronclark (license 1200)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@323371 65c4cc65-6c06-0410-ace0-fbb531ad65f3