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/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@418396 f38db490-d61c-443f-a65b-d21fe96a405b