Holding an unsaved body against its response id let a plugin stash the id
and read it in a later call, which would throw only sometimes and only
for ad-hoc sends. Documenting that was never going to be enough.
send now returns the response and its body together, so there is nothing
to stash: an unsaved body is a value you were handed. ctx.httpResponse
.body() goes back to meaning one thing, a saved response read by id, and
refuses ids it has no row for. Reading is identical either way, so no
caller has to know which kind of send it made.