res_pjsip_nat: Place source port into rport of responses if 'force_rport' is on.

When the 'force_rport' option is enabled the behavior should be the same
as if the remote side placed rport into the message themselves. Therefore
any responses we send should include the source port of the request in the
rport of the Via header.

#SIPit31

ASTERISK-24387 #close
Reported by: Matt Jordan


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@425131 f38db490-d61c-443f-a65b-d21fe96a405b