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/
........

Merged revisions 415359 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@415390 65c4cc65-6c06-0410-ace0-fbb531ad65f3