chan_sip: Fix order of variables specified in SIPNotify action

Prior to this patch, sequential variables would be ordered in reverse
from the order specified in the manager action.

Review: https://reviewboard.asterisk.org/r/3588/
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Merged revisions 415359 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 415390 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@415410 f38db490-d61c-443f-a65b-d21fe96a405b