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

pkg/json_rpc_2/test/server/utils.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.util;
 
 import 'package:unittest/unittest.dart';
 import 'package:json_rpc_2/json_rpc_2.dart' as json_rpc;
 
 void expectErrorResponse(json_rpc.Server server, request, int errorCode,
-    String message) {
+    String message, {data}) {
   var id;
   if (request is Map) id = request['id'];
+  if (data == null) data = {'request': request};
 
   expect(server.handleRequest(request), completion(equals({
     'jsonrpc': '2.0',
     'id': id,
     'error': {
       'code': errorCode,
       'message': message,
-      'data': {'request': request}
+      'data': data
     }
   })));
 }
 
 Matcher throwsInvalidParams(String message) {
   return throwsA(predicate((error) {
     expect(error, new isInstanceOf<json_rpc.RpcException>());
     expect(error.message, equals(message));
     return true;
   }));
 }