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/
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Merged revisions 403634 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 403635 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 403639 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403652 65c4cc65-6c06-0410-ace0-fbb531ad65f3