res_pjsip_refer: Channel variable SIPREFERTOHDR not being set during blind transfer

The SIPREFERTOHDR channel variable is not being set on any channel when
performing a blind transfer using PJSIP. The 'refer->refer_to' was not
being set during a blind transfer.  Updated so the 'refer_to' is set to
the target uri on a blind transfer.

(closes issue ASTERISK-23502)
Reported by: John Bigelow
Review: https://reviewboard.asterisk.org/r/3445/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@412453 f38db490-d61c-443f-a65b-d21fe96a405b