Include unit and distance_type when set

See #1836

Author: russcam

Diff for: src/Nest/CommonOptions/Sorting/SortJsonConverter.cs

@@ -65,7 +65,7 @@ public override void WriteJson(JsonWriter writer, object value, JsonSerializer s
 					break;
 				case "_geo_distance":
 					var geo = s as IGeoDistanceSort;
-					writer.WritePropertyName("_geo_distance");
+					writer.WritePropertyName(geo.SortKey.Name);
 					base.Reserialize(writer, s, serializer, w =>
 					{
 						writer.WritePropertyName(settings.Inferrer.Field(geo.Field));

Diff for: src/Nest/Search/Search/Sort/SortGeoDistance.cs

@@ -1,14 +1,19 @@
 using System;
 using System.Collections.Generic;
 using System.Linq.Expressions;
+using Newtonsoft.Json;
 
 namespace Nest
 {
 	public interface IGeoDistanceSort : ISort
 	{
 		Field Field { get; set; }
 		IEnumerable<GeoLocation> Points { get; set; }
+
+		[JsonProperty(PropertyName = "unit")]
 		DistanceUnit? GeoUnit { get; set; }
+
+		[JsonProperty(PropertyName = "distance_type")]
 		GeoDistanceType? DistanceType { get; set; }
 	}
 

Diff for: src/Tests/Search/Request/SortUsageTests.cs

@@ -39,17 +39,19 @@ public SortUsageTests(ReadOnlyCluster cluster, EndpointUsage usage) : base(clust
 					new {
 						_geo_distance = new {
 							location = new [] {
-							new {
-								lat = 70.0,
-								lon = -70.0
-							},
-							new {
-								lat = -12.0,
-								lon = 12.0
-							}
+								new {
+									lat = 70.0,
+									lon = -70.0
+								},
+								new {
+									lat = -12.0,
+									lon = 12.0
+								}
 							},
 							order = "asc",
-							mode = "min"
+							mode = "min",
+							distance_type = "arc",
+							unit = "cm"
 						}
 					},
 					new {
@@ -84,7 +86,7 @@ public SortUsageTests(ReadOnlyCluster cluster, EndpointUsage usage) : base(clust
 				)
 				.GeoDistance(g => g
 					.Field(p => p.Location)
-					.DistanceType(GeoDistanceType.SloppyArc)
+					.DistanceType(GeoDistanceType.Arc)
 					.Order(SortOrder.Ascending)
 					.Unit(DistanceUnit.Centimeters)
 					.Mode(SortMode.Min)