res_pjsip_header_funcs: Don't add headers to re-INVITEs.

When sending a re-INVITE to an endpoint it was possible for received
headers to be added as well (since they are stored for retrieval using
the PJSIP_HEADER dialplan function). This caused a broken (and
potentially large) SIP INVITE to be produced and sent.

This changes the module so it will no longer add headers to
re-INVITEs.

(closes issue ASTERISK-22882)
Reported by: David M. Lee
........

Merged revisions 403221 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403222 65c4cc65-6c06-0410-ace0-fbb531ad65f3