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

pkg/json_rpc_2/lib/src/exception.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.exception;
 
 import '../error_code.dart' as error_code;
 
 /// An exception from a JSON-RPC server that can be translated into an error
 /// response.
@@ skipping 34 matching lines @@
 
   /// Converts this exception into a JSON-serializable object that's a valid
   /// JSON-RPC 2.0 error response.
   serialize(request) {
     var modifiedData;
     if (data is Map && !data.containsKey('request')) {
       modifiedData = new Map.from(data);
       modifiedData['request'] = request;
     } else if (data == null) {
       modifiedData = {'request': request};
+    } else {
+      modifiedData = data;
     }
 
     var id = request is Map ? request['id'] : null;
     if (id is! String && id is! num) id = null;
     return {
       'jsonrpc': '2.0',
       'error': {'code': code, 'message': message, 'data': modifiedData},
       'id': id
     };
   }
 }