So, I notice that I dup data here, and that lutter does the same in registering it for the command log, but I doubt it does what you actually want -- it will preserve the top level hash, but *ONLY* the top level hash.  Mutations that change content below that hash will be visible in `old_data`.  For example, this will mutate old_data:

<pre>
{"tags": [{"name": "foo"}]} => {"tags": ["foo"]}
</pre>

Given that, I honestly doubt it is worth the headache of preserving the old data -- likely better, if you need access to it, to just use the raw JSON string from the body of the HTTP POST, no?