Yeah - we don't want a deep copy.  Actually that is (I think) already being achieved by `Mash#merge!` anyway.  What we really want is a deep *merge* - and I think I'll use `Chef::Mixin::DeepMerge::deep_merge!`.  (As I just mailed to the `crowbar` list, I'm slightly puzzled why you backported `hash_only_merge!` from Chef 11 for your "incomplete JSON" changes rather than using this.)