[System.ServiceModel.Web] Support `null` as value for `DateTime?` field.

Fixes assistly #5863.

mcs/class/System.ServiceModel.Web/System.Runtime.Serialization.Json/JsonSerializationReader.cs

@@ -72,9 +72,12 @@ namespace System.Runtime.Serialization.Json
 			if (serialized_object_count ++ == serializer.MaxItemsInObjectGraph)
 				throw SerializationError (String.Format ("The object graph exceeded the maximum object count '{0}' specified in the serializer", serializer.MaxItemsInObjectGraph));
 
-			if (type.IsGenericType && type.GetGenericTypeDefinition () == typeof (Nullable<>))
+			bool nullable = false;
+			if (type.IsGenericType && type.GetGenericTypeDefinition () == typeof (Nullable<>)) {
+				nullable = true;
 				type = Nullable.GetUnderlyingType (type);
-			
+			}
+
 			bool isNull = reader.GetAttribute ("type") == "null";
 
 			switch (Type.GetTypeCode (type)) {
@@ -122,8 +125,11 @@ namespace System.Runtime.Serialization.Json
 			case TypeCode.DateTime:
 				// it does not use ReadElementContentAsDateTime(). Different string format.
 				var s = reader.ReadElementContentAsString ();
-				if (s.Length < 2 || !s.StartsWith ("/Date(", StringComparison.Ordinal) || !s.EndsWith (")/", StringComparison.Ordinal))
+				if (s.Length < 2 || !s.StartsWith ("/Date(", StringComparison.Ordinal) || !s.EndsWith (")/", StringComparison.Ordinal)) {
+					if (nullable)
+						return null;
 					throw new XmlException ("Invalid JSON DateTime format. The value format should be '/Date(UnixTime)/'");
+				}
 
 				// The date can contain [SIGN]LONG, [SIGN]LONG+HOURSMINUTES or [SIGN]LONG-HOURSMINUTES
 				// the format for HOURSMINUTES is DDDD

mcs/class/System.ServiceModel.Web/Test/System.Runtime.Serialization.Json/DataContractJsonSerializerTest.cs

@@ -1385,6 +1385,17 @@ namespace MonoTests.System.Runtime.Serialization.Json
 			DateTest t = serializer.ReadObject(inputStream) as DateTest;
 			Assert.AreEqual (634129344000000000, t.ShouldHaveValue.Value.Ticks, "#1");
 		}
+
+		[Test]
+		public void NullableFieldsShouldSupportNullValue ()
+		{
+			string json = @"{""should_have_value"":null}";
+			var inputStream = new MemoryStream (Encoding.UTF8.GetBytes (json));
+			DataContractJsonSerializer serializer = new DataContractJsonSerializer(typeof(DateTest));
+			Console.WriteLine ("# serializer assembly: {0}", serializer.GetType ().Assembly.Location);
+			DateTest t = serializer.ReadObject (inputStream) as DateTest;
+			Assert.AreEqual (false, t.ShouldHaveValue.HasValue, "#2");
+		}
 		
 		[Test]
 		public void DeserializeNullMember ()