Allow SourceFiltering Disable to be set to false (#2496)

Fixes #2492

Author: russcam

src/Nest/Search/Search/SourceFiltering/SourceFilterJsonConverter.cs

@@ -19,9 +19,6 @@ public override void WriteJson(JsonWriter writer, object value, JsonSerializer s
 				writer.WriteValue(false);
 			else
 			{
-				if (filter.Include == null && filter.Exclude == null)
-					return;
-
 				writer.WriteStartObject();
 				if (filter.Include != null)
 				{

src/Tests/Search/Request/SourceFilteringUsageTests.cs

@@ -10,8 +10,8 @@
 namespace Tests.Search.Request
 {
 	/** Allows to control how the `_source` field is returned with every hit.
-	 * By default operations return the contents of the `_source` field unless
-	 * you have used the fields parameter or if the `_source` field is disabled.
+	 * By default, operations return the contents of the `_source` field unless
+	 * you have used the fields parameter, or if the `_source` field is disabled.
 	 *
 	 * See the Elasticsearch documentation on {ref_current}/search-request-source-filtering.html[Source Filtering] for more detail.
 	 */
@@ -97,6 +97,34 @@ protected override void ExpectResponse(ISearchResponse<Project> response)
 		}
 	}
 
+	//hide
+	public class SourceFilteringEnabledUsageTests : SearchUsageTestBase
+	{
+		public SourceFilteringEnabledUsageTests(ReadOnlyCluster cluster, EndpointUsage usage) : base(cluster, usage) { }
+
+		protected override object ExpectJson =>
+			new
+			{
+				_source = new {}
+			};
+
+		protected override Func<SearchDescriptor<Project>, ISearchRequest> Fluent => s => s
+			.Source();
+
+		protected override SearchRequest<Project> Initializer =>
+			new SearchRequest<Project>
+			{
+				Source = new SourceFilter()
+			};
+
+		protected override void ExpectResponse(ISearchResponse<Project> response)
+		{
+			response.ShouldBeValid();
+			foreach (var hit in response.Hits)
+				hit.Source.Should().NotBeNull();
+		}
+	}
+
 	//hide
 	public class SourceFilteringSerializationTests : SerializationTestBase
 	{