UpdateIndexSettingsRequest does not work on adding well-known settings through Dictionary.Add() method

[bittusarkar]

NEST/Elasticsearch.Net version: 5.6.0

Elasticsearch version: 5.4.0

Description of the problem including expected versus actual behavior:
I'm trying to write a generic code for updating index setting where the setting name and value are provided as inputs. If I initialize an object of IndexSettings class and add a setting using the Dictionary.Add() method for a well-known setting, then Nest throws saying "no settings to update".

Steps to reproduce:

var settings = new IndexSettings();
settings.Add("index.unassigned.node_left.delayed_timeout", "10m");
var request = new UpdateIndexSettingsRequest(Indices.AllIndices)
{
    IndexSettings = settings
};
client.UpdateIndexSettings(request);

I understand that using settings.UnassignedNodeLeftDelayedTimeout = "10m"; works but when I do not know what the setting is beforehand, I cannot use this. I also understand that I can achieve this using the low level client but I would like to avoid that as much as possible.

My ask is to allow configuring any setting by adding to a dictionary irrespective of whether it is a well-known setting or not.

[hyankov]

This doesn't work for me, as I want to return refresh_interval to its default value (without knowing what the default is for the cluster!)

client.UpdateIndexSettings(new UpdateIndexSettingsRequest("some_index_name") { IndexSettings = new IndexSettings() { RefreshInterval = null } });

generates PUT request with empty body:
https://cluster/some_index_name/_settings?pretty=true

This gets rejected with:

{
  "error" : {
    "root_cause" : [ {
      "type" : "action_request_validation_exception",
      "reason" : "Validation Failed: 1: no settings to update;"
    } ],
    "type" : "action_request_validation_exception",
    "reason" : "Validation Failed: 1: no settings to update;"
  },
  "status" : 400
}

It's line 26 of IndexSettingsConverter.cs in the change:
if (newValue != null) d[knownKey] = newValue;

Look at the documentation of ElasticSearch:

To reset a setting back to the default value, use null. For example:

PUT /twitter/_settings
{
"index" : {
"refresh_interval" : null
}
}