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

pkg/json_rpc_2/test/server/server_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.server_test;
 
 import 'dart:convert';
 
 import 'package:unittest/unittest.dart';
 import 'package:json_rpc_2/error_code.dart' as error_code;
@@ skipping 74 matching lines @@
   test("doesn't return a result for a notification", () {
     server.registerMethod('foo', (args) => 'result');
 
     expect(server.handleRequest({
       'jsonrpc': '2.0',
       'method': 'foo',
       'params': {}
     }), completion(isNull));
   });
 
+  test("includes the error data in the response", () {
+    server.registerMethod('foo', (params) {
+      throw new json_rpc.RpcException(5, 'Error message.', data: 'data value');
+    });
+
+    expectErrorResponse(server, {
+      'jsonrpc': '2.0',
+      'method': 'foo',
+      'params': {},
+      'id': 1234
+    },
+        5,
+        'Error message.',
+        data: 'data value');
+  });
+
   group("JSON", () {
     test("handles a request parsed from JSON", () {
       server.registerMethod('foo', (params) {
         return {'params': params.value};
       });
 
       expect(server.parseRequest(JSON.encode({
         'jsonrpc': '2.0',
         'method': 'foo',
         'params': {'param': 'value'},
@@ skipping 89 matching lines @@
         }
       }));
     });
   });
 
   test("disallows multiple methods with the same name", () {
     server.registerMethod('foo', () => null);
     expect(() => server.registerMethod('foo', () => null), throwsArgumentError);
   });
 }