Deserializes req_json as JSON, invokes self.call(), and serializes result to JSON.
        Returns JSON encoded string.

        :Parameters:
          req_json
            JSON-RPC request serialized as JSON string
          props
            Application defined properties to set on RequestContext for use with filters. 
            For example: authentication headers.  Must be a dict.

def call_json(self, req_json, props=None):
        """
        Deserializes req_json as JSON, invokes self.call(), and serializes result to JSON.
        Returns JSON encoded string.

        :Parameters:
          req_json
            JSON-RPC request serialized as JSON string
          props
            Application defined properties to set on RequestContext for use with filters. 
            For example: authentication headers.  Must be a dict.
        """
        try:
            req = json.loads(req_json)
        except:
            msg = "Unable to parse JSON: %s" % req_json
            return json.dumps(err_response(None, -32700, msg))
        return json.dumps(self.call(req, props))