Fix issue where session timer headers were present when they should not have been.

(closes issue #12706)
Reported by: falves11
Patches:
      chan_sip.c.diff uploaded by rjain (license 226)
Tested by: falves11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@121503 65c4cc65-6c06-0410-ace0-fbb531ad65f3