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.
This looks like a good change.  I've seen lot of places in the RPC code that are using raw char[] where we could be using members of the `jss` namespace.  I started "fixing" them but got into a rats nest.  Do you have a sense for which strings belong in `jss` and which ones don't (other than use the ones that are already there)?