Move evaluation of set_var options in pjsip to the end of channel initialization.

This allows for set_var to override certain defaults such as caller ID and codec
values. This also fixes a test suite regression. The "set_var" test suite test attempted
to use set_var to override caller ID, but a recent change caused that to no longer work.
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Merged revisions 421565 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 421566 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@421567 65c4cc65-6c06-0410-ace0-fbb531ad65f3