Make JSON RPC v2.0 schema reject unknown properties

This was easy for the Request, but the Response required a bit of re-organization to remove the "allOf" which doesn't allow an easy way of setting "additionalProperties": false

I've tested these changes against two different implementations of jsonrpc v2.0 so hopefully they are OK!

Rejecting unknown properties is desirable for exact protocol conformance, even though most clients and servers probably don't care.