Add RequestParams overload without optional content type

[russcam]

This commit adds back a RequestParams overload on NamespacedClientProxy
in Elasticsearch.Net that does not accept a content type
optional parameter, and removes the optional part
of the parameter on the other overload, for backwards
binary compatibility.

[Mpdreamz]

Lgtm

[Mpdreamz]

++

[Mpdreamz]

So close to a green build:

[xUnit.net 00:05:16.6990755]     Tests.Indices.IndexManagement.SplitIndex.SplitIndexApiTests.ReturnsExpectedResponse [FAIL]
  X Tests.Indices.IndexManagement.SplitIndex.SplitIndexApiTests.ReturnsExpectedResponse [2s 995ms]
  Error Message:
   Tests.Framework.EndpointTests.ResponseAssertionException : Expected boolean to be true, but found False.
Response Under Test:
Valid NEST response built from a successful (200) low level call on PUT: /nest-fluentasync-bed1278b/_split/nest-fluentasync-bed1278b-target?pretty=true&error_trace=true
# Audit trail of this API call:
 - [1] HealthyResponse: Node: http://localhost:9200/ Took: 00:00:31.1440714
# Request:
{"settings":{"index.number_of_shards":8}}
# Response:
{
  "acknowledged" : true,
  "shards_acknowledged" : false,
  "index" : "nest-fluentasync-bed1278b-target"
}

shards_acknowledged: false

[russcam]

😬