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.
The changes you've made look pretty reasonable.  Unfortunately there's no way for the compiler to help enforce that a Json::Object is the only type returned.  So as the code is maintained in the future someone might slip up.  I suggest that you at least add an `assert (jvRaw.isObject ())` here with a comment so we have a run-time check.