Fix ability to perform a remote attended transfer with PJSIP.

This fix has two parts:

* Corrected an error message to properly state that external_replaces is an extension. The
  error message also prints what dialplan context the external_replaces extension was being
  looked for in.
* Corrected the printing of the Replaces: header in an INVITE request. We were duplicating
  "Replaces: " in the header.

ASTERISK-24376 #close
Reported by Matt Jordan

Review: https://reviewboard.asterisk.org/r/4296
........

Merged revisions 430313 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@430314 65c4cc65-6c06-0410-ace0-fbb531ad65f3