Pass RpcException error data through if it's not a map.

pkg/json_rpc_2/test/server/parameters_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library json_rpc_2.test.server.parameters_test;
 
 import 'package:unittest/unittest.dart';
-import 'package:json_rpc_2/error_code.dart' as error_code;
 import 'package:json_rpc_2/json_rpc_2.dart' as json_rpc;
 
 import 'utils.dart';
 
 void main() {
   group("with named parameters", () {
     var parameters;
     setUp(() {
       parameters = new json_rpc.Parameters("foo", {
         "num": 1.5,
@@ skipping 348 matching lines @@
   test("with a complex parameter path", () {
     var parameters = new json_rpc.Parameters("foo", {
       'bar baz': [0, 1, 2, {'bang.zap': {'\n': 'qux'}}]
     });
 
     expect(() => parameters['bar baz'][3]['bang.zap']['\n']['bip'],
         throwsInvalidParams('Parameter "bar baz"[3]."bang.zap"."\\n" for '
             'method "foo" must be an Object, but was "qux".'));
   });
 }