[Catalog SDK] Fix serialization issue (#671)

Mirrors NuGet/Samples#34, discovered by @jstoker

src/NuGet.Protocol.Catalog/Serialization/BaseCatalogLeafConverter.cs

@@ -27,6 +27,7 @@ public override void WriteJson(JsonWriter writer, object value, JsonSerializer s
             if (_fromType.TryGetValue((CatalogLeafType)value, out output))
             {
                 writer.WriteValue(output);
+                return;
             }
 
             throw new NotSupportedException($"The catalog leaf type '{value}' is not supported.");

tests/NuGet.Protocol.Catalog.Tests/Models/CatalogLeafItemFacts.cs

@@ -0,0 +1,29 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Newtonsoft.Json;
+using Xunit;
+
+namespace NuGet.Protocol.Catalog
+{
+    public class CatalogLeafItemFacts
+    {
+        [Fact]
+        public void JsonSerializationException()
+        {
+            var leafItem = new CatalogLeafItem
+            {
+                Url = "https://api.nuget.org/v3/registration3-gz-semver2/newtonsoft.json/12.0.1.json",
+                Type = CatalogLeafType.PackageDetails,
+                CommitTimestamp = DateTimeOffset.Parse("2019-10-10T00:00:00.00+00:00"),
+                PackageId = "Newtonsoft.Json",
+                PackageVersion = "12.0.1"
+            };
+
+            var result = JsonConvert.SerializeObject(leafItem);
+
+            Assert.Equal(TestData.CatalogLeafItem, result);
+        }
+    }
+}

tests/NuGet.Protocol.Catalog.Tests/NuGet.Protocol.Catalog.Tests.csproj

@@ -36,6 +36,7 @@
   <ItemGroup>
     <Compile Include="CatalogClientFacts.cs" />
     <Compile Include="CatalogProcessorSettingsFacts.cs" />
+    <Compile Include="Models\CatalogLeafItemFacts.cs" />
     <Compile Include="Models\ModelExtensionsFacts.cs" />
     <Compile Include="NullLogger.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />