Fixed support of batch rpc calls

According to [Flask Docs](http://flask.pocoo.org/docs/security/#json-security) it's not secure to respond a JSON with top-level array. But according to [JSON-RPC 2.0 Specification](http://www.jsonrpc.org/specification#batch) during batch requests it is possible to return array of Response objects. So, I've mocked flask.jsonify function in helper.