Don't replace instance.info_cache on each save

If we are working on an Instance that came from an older client,
we should not replace an older nested .info_cache object with a
newer one for no reason during a save().

Related-Bug: #1258256
Change-Id: I8cf4a629d454de1a5de16ea4032f39e36aab505a
Merge "Don't replace instance.info_cache on each save"