Fix schema generation of sealed type with extension data (#332)

Src/Newtonsoft.Json.Schema.Tests/Issues/Issue0331Tests.cs

@@ -0,0 +1,45 @@
+#region License
+// Copyright (c) Newtonsoft. All Rights Reserved.
+// License: https://raw.github.com/JamesNK/Newtonsoft.Json.Schema/master/LICENSE.md
+#endregion
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using Newtonsoft.Json.Linq;
+using Newtonsoft.Json.Schema.Generation;
+using Newtonsoft.Json.Serialization;
+#if DNXCORE50
+using Xunit;
+using Test = Xunit.FactAttribute;
+using Assert = Newtonsoft.Json.Schema.Tests.XUnitAssert;
+#else
+using NUnit.Framework;
+#endif
+
+namespace Newtonsoft.Json.Schema.Tests.Issues
+{
+    [TestFixture]
+    public class Issue0331Tests : TestFixtureBase
+    {
+        [Test]
+        public void Test()
+        {
+            JSchemaGenerator generator = new JSchemaGenerator();
+            JSchema schema = generator.Generate(typeof(MyDto));
+
+            Assert.IsFalse(schema.AllowAdditionalPropertiesSpecified);
+        }
+
+        public sealed class MyDto
+        {
+            [JsonProperty("foo")]
+            public string Foo { get; set; }
+
+            [JsonExtensionData]
+            public IDictionary<string, JToken> ExtensionData { get; set; }
+        }
+    }
+}

Src/Newtonsoft.Json.Schema/Generation/JSchemaGeneratorInternal.cs

@@ -642,7 +642,7 @@ private void GenerateObjectSchema(JSchema schema, Type type, JsonObjectContract
                 }
             }
 
-            if (type.IsSealed())
+            if (type.IsSealed() && contract.ExtensionDataGetter == null)
             {
                 schema.AllowAdditionalProperties = false;
             }