Fix Setting A chan_sip Dialog's SIP_NAT_FORCE_RPORT Flag

A condition was added in a commit to fix ASTERISK-21374, that, if the
SIP_PAGE3_NAT_AUTO_RPORT flag was set, to then copy a peer's SIP_NAT_FORCE_RPORT
flag to the dialog.  This condition should not have been there since it assumed
that if Asterisk is in an environment where NAT is involved, that the auto_* nat
settings or force_rport setting would be on in the global settings.  If the nat
setting in the global setting is set to 'nat=no' and then turned on for peers
(which is not quite the recommended way, although it is allowed) this flag is
never copied to the dialog resulting in problems like, REGISTER replies going
to the wrong port.

This patch removes this conditional check and will now always use the peer's
flag which by this point in the code the checks on whether the peer is behind
NAT or not (if using auto_force_rport) have already been run.

(closes issue ASTERISK-22236)
Reported by: Filip Frank
Tested by: Michael L. Young
Patches:
    asterisk-2236-always-set-rport.diff uploaded
                                              by Michael L. Young (license 5026)

Review: https://reviewboard.asterisk.org/r/2919/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@401167 f38db490-d61c-443f-a65b-d21fe96a405b