TokenFilters configured on IndexSettings are serialized incorrectly

[stevejgordon]

Generated code for open internally-tagged unions produces incorrect JSON when serialized. An example is when defining token filters on IndexSettings.

var createResponse = await client.Indices.CreateAsync("my-new-index", i => i
    .Settings(s => s
        .Analysis(a => a
            .Filter(f => f
                .Add("synonym", new TokenFilter(new TokenFilterDefinitions
                    {
                        { "synonym", new SynonymTokenFilter() { SynonymsPath = "analysis/synonym.txt" } }
                    })))
            .Analyzer(a => a
                .Custom("my-custom-analyzer", c => c
                    .Filter(new[] { "stop", "synonym" })
                    .Tokenizer("standard"))))));

The above produces:

{
    "settings": {
        "analysis": {
            "analyzer": {
                "my-custom-analyzer": {
                    "filter": ["stop", "synonym"],
                    "tokenizer": "standard",
                    "type": "custom"
                }
            },
            "filter": {
                "synonym": {
                    "synonym": {
                        "synonyms_path": "analysis/synonym.txt",
                        "type": "synonym"
                    }
                }
            }
        }
    }
}

We expect:

{
    "settings": {
        "analysis": {
            "analyzer": {
                "my-custom-analyzer": {
                    "filter": ["stop", "synonym"],
                    "tokenizer": "standard",
                    "type": "custom"
                }
            },
            "filter": {
                "synonym": {
                    "synonyms_path": "analysis/synonym.txt",
                    "type": "synonym"
                }
            }
        }
    }
}

This is due to the generation of TokenFilterDefinitions as IsADictionary required for this polymorphic type's open nature. This will affect other similarly specified types.