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.
........

Merged revisions 421565 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@421566 f38db490-d61c-443f-a65b-d21fe96a405b