Copy CDR variables when set during a bridge

This patch makes sure amaflags, accountcode, and userfield get copied
to the bridge CDR when set during a bridge (like via a custom feature).

(closes issue ASTERISK-16990)
Review: https://reviewboard.asterisk.org/r/1721/
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Merged revisions 356963 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 356964 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@356965 65c4cc65-6c06-0410-ace0-fbb531ad65f3