Make res_sorcery_realtime filter unknown retrieved results.

When retrieving data from a database or other realtime backend, it's quite
possible to retrieve variables that Asterisk does not care about but that
are legitimate to exist. Asterisk does not need to throw a hissy fit when
these variables are encountered but rather just filter them out.

Review: https://reviewboard.asterisk.org/r/3305
........

Merged revisions 410187 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@410207 65c4cc65-6c06-0410-ace0-fbb531ad65f3