Serialize null values in composite aggregation key (#3605)

Fixes #3575

Author: russcam

src/Nest/Aggregations/Bucket/Composite/CompositeBucket.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using Newtonsoft.Json;
 
 namespace Nest
 {
@@ -25,6 +26,7 @@ public CompositeBucket(IReadOnlyDictionary<string, IAggregate> dict, CompositeKe
 	/// <summary>
 	/// A key for a <see cref="CompositeBucket" />
 	/// </summary>
+	[JsonConverter(typeof(VerbatimDictionaryKeysPreservingNullJsonConverter<string, object>))]
 	public class CompositeKey : IsAReadOnlyDictionaryBase<string, object>
 	{
 		private static readonly DateTimeOffset Epoch = new DateTimeOffset(1970, 1, 1, 0, 0, 0, 0, TimeSpan.Zero);

src/Tests/Tests/Framework/SerializationTests/CompositeKeySerializationTests.cs

@@ -0,0 +1,36 @@
+using System.Collections.Generic;
+using System.IO;
+using System.Text;
+using Elastic.Xunit.XunitPlumbing;
+using Elasticsearch.Net;
+using FluentAssertions;
+using Nest;
+using Tests.Core.Client;
+using Tests.Core.Extensions;
+
+namespace Tests.Framework.SerializationTests
+{
+	public class CompositeKeySerializationTests
+	{
+		[U] public void NullValuesAreSerialized()
+		{
+			var compositeKey = new CompositeKey(new Dictionary<string, object>
+			{
+				{ "key_1", "value_1" },
+				{ "key_2", null },
+			});
+
+			var serializer = TestClient.Default.RequestResponseSerializer;
+			var json = serializer.SerializeToString(compositeKey, SerializationFormatting.None);
+			json.Should().Be("{\"key_1\":\"value_1\",\"key_2\":null}");
+
+			using (var stream = new MemoryStream(Encoding.UTF8.GetBytes(json)))
+			{
+				stream.Position = 0;
+				var dictionary = serializer.Deserialize<IReadOnlyDictionary<string, object>>(stream);
+				var deserializedCompositeKey = new CompositeKey(dictionary);
+				compositeKey.Should().Equal(deserializedCompositeKey);
+			}
+		}
+	}
+}