Make sure that handlers always return Json::objectValue.

Reviewers: @nbougalis @scottschurr

This is necessary for the "streaming JSON RPC" review upcoming, but it's also useful in its own right.
I don't know exclusively.

But anything that is used to index a Json::Value that's then incorporated into a handler result definitely should be a `jss` value.  I clean them up as I see them, if it's relevant...