HandleNestTypesOnSourceJsonConverter does not serialize/deserialize the join field as expected

NEST/Elasticsearch.Net version: 6.2.0

Elasticsearch version: 6.x

Description of the problem including expected versus actual behavior:

HandleNestTypesOnSourceJsonConverter does not serialize/deserialize the join field as expected.

"myparent" vs "\"myparent\""

Steps to reproduce:

1. Check attached source code

            var parentDocument = new MyParent
            {
                Id = 1,
                ParentProperty = "a parent prop",
                MyJoinField = "myparent"
            };

            var settings = new JsonSerializerSettings();
            settings.Converters.Add(new HandleNestTypesOnSourceJsonConverter(((IConnectionSettingsValues)connectionSettings).RequestResponseSerializer));

            var json01 = Newtonsoft.Json.JsonConvert.SerializeObject(parentDocument, Formatting.Indented, settings);
            var back01 = Newtonsoft.Json.JsonConvert.DeserializeObject<MyParent>(json01, settings);

            var json02 = Newtonsoft.Json.JsonConvert.SerializeObject(back01, Formatting.Indented, settings);

            Console.WriteLine($"JSON01: {json01}");
            Console.WriteLine($"JSON02: {json02}");

2. Result

    JSON01: {
        "ParentProperty": "a parent prop",
        "Id": 1,
        "MyJoinField": "myparent"
    }
    JSON02: {
        "ParentProperty": "a parent prop",
        "Id": 1,
        "MyJoinField": "\"myparent\""
    }

Note: This is causing a lot of troubles for as as we use to serialize/deserialize to send documents between micro services.

Here the source code.

Program.txt

[russcam]

Thanks for reporting @badpp, I've opened #3357 to address

Thanks!

Created a followup issue as it is still in v6.3.0 (there will be no version v.6.2.1), see #3403