Support non string values for sort missing

Closes #2857

Author: russcam

src/Nest/Search/Search/Sort/SortBase.cs

@@ -11,7 +11,7 @@ public interface ISort
 		Field SortKey { get; }
 
 		[JsonProperty("missing")]
-		string Missing { get; set; }
+		object Missing { get; set; }
 
 		[JsonProperty("order")]
 		SortOrder? Order { get; set; }
@@ -28,7 +28,7 @@ public interface ISort
 
 	public abstract class SortBase : ISort
 	{
-		public string Missing { get; set; }
+		public object Missing { get; set; }
 		public SortOrder? Order { get; set; }
 		public SortMode? Mode { get; set; }
 		public QueryContainer NestedFilter { get; set; }
@@ -37,14 +37,14 @@ public abstract class SortBase : ISort
 		protected abstract Field SortKey { get; }
 	}
 
-	public abstract class SortDescriptorBase<TDescriptor, TInterface, T> : DescriptorBase<TDescriptor, TInterface>, ISort 
-		where T : class 
+	public abstract class SortDescriptorBase<TDescriptor, TInterface, T> : DescriptorBase<TDescriptor, TInterface>, ISort
+		where T : class
 		where TDescriptor : SortDescriptorBase<TDescriptor, TInterface, T>, TInterface, ISort
 		where TInterface : class, ISort
 	{
 		Field ISort.SortKey => this.SortKey;
 
-		string ISort.Missing { get; set; }
+		object ISort.Missing { get; set; }
 
 		SortOrder? ISort.Order { get; set; }
 
@@ -75,7 +75,7 @@ public virtual TDescriptor NestedFilter(Func<QueryContainerDescriptor<T>, QueryC
 
 		public virtual TDescriptor MissingFirst() => Assign(a => a.Missing = "_first");
 
-		public virtual TDescriptor MissingValue(string value) => Assign(a => a.Missing = value);
+		public virtual TDescriptor Missing(object value) => Assign(a => a.Missing = value);
 
 	}
-}
+}

src/Tests/Search/Request/SortUsageTests.cs

@@ -37,6 +37,12 @@ public SortUsageTests(ReadOnlyCluster cluster, EndpointUsage usage) : base(clust
 							unmapped_type = "date"
 						}
 					},
+					new {
+						numberOfCommits = new {
+							missing = -1,
+							order = "desc"
+						}
+					},
 					new {
 						_geo_distance = new {
 							location = new [] {
@@ -85,6 +91,11 @@ public SortUsageTests(ReadOnlyCluster cluster, EndpointUsage usage) : base(clust
 					.NestedPath(p => p.Tags)
 					.NestedFilter(q => q.MatchAll())
 				)
+				.Field(f => f
+					.Field(p => p.NumberOfCommits)
+					.Order(SortOrder.Descending)
+					.Missing(-1)
+				)
 				.GeoDistance(g => g
 					.Field(p => p.Location)
 					.DistanceType(GeoDistanceType.Arc)
@@ -122,6 +133,12 @@ public SortUsageTests(ReadOnlyCluster cluster, EndpointUsage usage) : base(clust
 						NestedPath = Field<Project>(p=>p.Tags),
 						NestedFilter = new MatchAllQuery(),
 					},
+					new SortField
+					{
+						Field = Field<Project>(p=>p.NumberOfCommits),
+						Order = SortOrder.Descending,
+						Missing = -1
+					},
 					new GeoDistanceSort
 					{
 						Field = "location",