-Fixed schema generation with negative enum values

Src/Newtonsoft.Json.Tests/Schema/JsonSchemaGeneratorTests.cs

@@ -449,5 +449,54 @@ namespace Newtonsoft.Json.Tests.Schema
   }
 }", json);
     }
+
+    public enum SortTypeFlag
+    {
+      No = 0,
+      Asc = 1,
+      Desc = -1
+    }
+
+    public class X
+    {
+      public SortTypeFlag x;
+    }
+
+    [Test]
+    public void GenerateSchemaWithNegativeEnum()
+    {
+      JsonSchemaGenerator jsonSchemaGenerator = new JsonSchemaGenerator();
+      JsonSchema schema = jsonSchemaGenerator.Generate(typeof(X));
+
+      string json = schema.ToString();
+
+      Assert.AreEqual(@"{
+  ""type"": ""object"",
+  ""properties"": {
+    ""x"": {
+      ""type"": ""integer"",
+      ""enum"": [
+        0,
+        1,
+        -1
+      ],
+      ""options"": [
+        {
+          ""value"": 0,
+          ""value"": ""No""
+        },
+        {
+          ""value"": 1,
+          ""value"": ""Asc""
+        },
+        {
+          ""value"": -1,
+          ""value"": ""Desc""
+        }
+      ]
+    }
   }
-}
+}", json);
+    }
+  }
+}

Src/Newtonsoft.Json/Schema/JsonSchemaGenerator.cs

@@ -311,8 +311,8 @@ namespace Newtonsoft.Json.Schema
           CurrentSchema.Enum = new List<JToken>();
           CurrentSchema.Options = new Dictionary<JToken, string>();
 
-          EnumValues<ulong> enumValues = EnumUtils.GetNamesAndValues<ulong>(type);
-          foreach (EnumValue<ulong> enumValue in enumValues)
+          EnumValues<long> enumValues = EnumUtils.GetNamesAndValues<long>(type);
+          foreach (EnumValue<long> enumValue in enumValues)
           {
             JToken value = JToken.FromObject(enumValue.Value);