Fix null value handling on dictionaries

Null values within dictionaries were being ignored even with
NullValueHandling set to Include on the serializer.

Closes #1004

src/Nest/Resolvers/Converters/DictionaryKeysAreNotPropertyNamesJsonConverter.cs

@@ -36,7 +36,7 @@ public override void WriteJson(JsonWriter writer, object value, JsonSerializer s
 
 			foreach (DictionaryEntry entry in dictionary)
 			{
-				if (entry.Value == null)
+				if (entry.Value == null && serializer.NullValueHandling == NullValueHandling.Ignore)
 					continue;
 				string key;
 				var pp = entry.Key as PropertyPathMarker;

src/Tests/Nest.Tests.Unit/Internals/Serialize/NullValueHandlingTests.cs

@@ -0,0 +1,102 @@
+using Elasticsearch.Net.Connection;
+using Nest.Tests.MockData.Domain;
+using Newtonsoft.Json;
+using NUnit.Framework;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Nest.Tests.Unit.Internals.Serialize
+{
+	public class PartialDoc
+	{
+		public string Name { get; set; }
+		public DateTime? StartedOn { get; set; }
+	}
+
+	[TestFixture]
+	public class NullValueHandlingTests : BaseJsonTests
+	{
+		[Test]
+		public void IncludeNullValues_Dictionary_Test()
+		{
+			var client = GetTestClient(NullValueHandling.Include);
+
+			var doc = new Dictionary<string, object>
+			{
+				{ "name", "newname" },
+				{ "startedOn", null }
+			};
+
+			var expected = @"{ name: ""newname"", startedOn: null }";
+			var json = Encoding.UTF8.GetString(client.Serializer.Serialize(doc));
+
+			Assert.True(json.JsonEquals(expected), json);
+		}
+
+		[Test]
+		public void IncludeNullValues_Object_Test()
+		{
+			var client = GetTestClient(NullValueHandling.Include);
+
+			var doc = new PartialDoc
+			{
+				Name = "newname",
+				StartedOn = null
+			};
+
+			var expected = @"{ name: ""newname"", startedOn: null }";
+			var json = Encoding.UTF8.GetString(client.Serializer.Serialize(doc));
+
+			Assert.True(json.JsonEquals(expected), json);
+		}
+
+		[Test]
+		public void IgnoreNullValues_Dictionary_Test()
+		{
+			var client = GetTestClient(NullValueHandling.Ignore);
+
+			var doc = new Dictionary<string, object>
+			{
+				{ "name", "newname" },
+				{ "startedOn", null }
+			};
+
+			var expected = @"{ name: ""newname"" }";
+			var json = Encoding.UTF8.GetString(client.Serializer.Serialize(doc));
+
+			Assert.True(json.JsonEquals(expected), json);
+		}
+
+		[Test]
+		public void IgnoreNullValues_Object_Test()
+		{
+			var client = GetTestClient(NullValueHandling.Ignore);
+
+			var doc = new PartialDoc
+			{
+				Name = "newname",
+				StartedOn = null
+			};
+
+			var expected = @"{ name: ""newname"" }";
+			var json = Encoding.UTF8.GetString(client.Serializer.Serialize(doc));
+
+			Assert.True(json.JsonEquals(expected), json);
+		}
+
+		private IElasticClient GetTestClient(NullValueHandling nullValueHandling)
+		{
+			var settings = new ConnectionSettings(UnitTestDefaults.Uri, UnitTestDefaults.DefaultIndex)
+				.SetJsonSerializerSettingsModifier(m => m
+					.NullValueHandling = nullValueHandling
+				);
+			var connection = new InMemoryConnection(settings);
+			var client = new ElasticClient(settings, connection);
+
+			return client;
+		}
+	}
+}

src/Tests/Nest.Tests.Unit/Nest.Tests.Unit.csproj

@@ -237,6 +237,7 @@
     <Compile Include="Internals\Inferno\HostNameWithPathTests.cs" />
     <Compile Include="Internals\Inferno\MapTypeNamesTests.cs" />
     <Compile Include="Internals\Serialize\ConnectionSettingsTests.cs" />
+    <Compile Include="Internals\Serialize\NullValueHandlingTests.cs" />
     <Compile Include="Internals\Serialize\OptOutTests.cs" />
     <Compile Include="Internals\Serialize\CustomConvertersTests.cs" />
     <Compile Include="ObjectInitializer\Aliases\GetAliasMoreUrlTests.cs" />