fixed jsonrpc response 0 not handled properly

lib/jsonrpc.js

@@ -45,7 +45,7 @@ var isValidResponse = function (response) {
         !response.error &&
         response.jsonrpc === '2.0' &&
         typeof response.id === 'number' &&
-        (!!response.result || typeof response.result === 'boolean');
+        response.result !== undefined; // only undefined is not valid json object
 };
 
 /// Should be called to create batch payload object