tests in one place
KV#last_request_time
KeyValue#destroy[!?] unsets the ref

The thinking here:

1) Delete sets the value of the key on the service to a null object

2) #save[!?] normally sends If-Match with the value of @ref

3) The key still exists, you can get the full ref history, etc.  You can
re-create it, and keepings its value around may prove useful.

4) If you call #save again on the deleted KV, it will save without the
'If-Match' header