astobj2: tweak ao2_replace to do nothing when it would be a NoOp

This change causes ao2_replace to do nothing when src == dst. This
avoids REF_DEBUG logging when we're not actually doing anything.

Review: https://reviewboard.asterisk.org/r/3743/
........

Merged revisions 418396 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@418397 65c4cc65-6c06-0410-ace0-fbb531ad65f3