Log the userfield CDR variable like the other CDR backends, assuming the

column is actually there.  If it's not, we still log everything else as
before.

(closes issue #13281)
Reported by: falves11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@137203 65c4cc65-6c06-0410-ace0-fbb531ad65f3