Reset peer outboundproxy on sip.conf reload

If you set a peer's outboundproxy and then removed it from the config,
this would not get picked up in a config reload.  This patch fixes that
by resetting it in set_peer_defaults().

Closes ASTERISK-19454
Review: https://reviewboard.asterisk.org/r/3065/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@403634 f38db490-d61c-443f-a65b-d21fe96a405b