Add capability to remove added SIP headers *before* INVITE is generated.

(closes issue #14246)
Reported by: klaus3000
Patches: 
      2patch_chan_sip_SIPRemoveHeader_trunk.txt uploaded by klaus3000 (license 65)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@168638 65c4cc65-6c06-0410-ace0-fbb531ad65f3