PostData<T> does not work correctly except in string case

[godefroi]

In es-net 2.0.0-rc1, I have found that using PostData<T> in various scenarios resulted in odd content being posted, as verified by Wireshark. I can update this issue tomorrow with the specific examples of the data, but the only scenario that I could verify as working is the PostData<string> case. When the type is a serializable type or byte[], the correct data is not posted to the server. I did not try the IEnumerable<T> cases.

[Mpdreamz]

We had a pretty extensive unit test suite for PostData<> but no real integration tests. Added this and it uncovered it didn't handle boxed types very well. In the IEnumerable case it should also force no formatting.

Again thanks for opening this issue @godefroi much much appreciated 👍

[godefroi]

PostData<T> still doesn't work correctly in the PostType.Serializable case. I get the following data sent over the wire to ES:

POST /unittests_record_cdooacwg55xeflhn3hxzkpjtae/record/rec.1.2.3%5Crecc?pretty=true HTTP/1.1
Accept: application/json
Content-Type: application/json
Host: 10.8.16.195:9200
Content-Length: 30

{"WrittenBytes":null,"Type":4}

Clearly it's the PostData<T> object that's being serialized, not the object that it's wrapping. This is with 2.0.1.

[godefroi]

More information: Passing a PostData<Record> into ElasticLowLevelClient.Index() results in the object being cast to PostData<object>, which doesn't work as expected. Passing my Record object directly works, because of the implicit operator on PostData<T>, but passing an actual instance of PostData<T> where T is not object does not work as expected. Instead of the method signature being:
Index<T>(string index, string type, string id, PostData<object> body, Func<IndexRequestParameters, IndexRequestParameters> requestParameters = null)

It should probably be:
Index<T>(string index, string type, string id, PostData<T> body, Func<IndexRequestParameters, IndexRequestParameters> requestParameters = null)

This would eliminate the cast.