MapIdPropertyFor field not being used when null Id passed in

[spresnal]

NEST/Elasticsearch.Net version: 5.x

Elasticsearch version: 5.2

Description of the problem including expected versus actual behavior:
When I pass a null Id into the 1.x NEST client, the client overwrites the value with the MapIdPropertyFor property instead of passing null to ElasticSearch.

When I pass a null Id into the 5.x NEST client, the client does not overwrite the value with the MapIdPropertyFor property and passes null to ElasticSearch.

Steps to reproduce:

1. Add MapIdPropertyFor to ConnectionSettings
2. Index a document with the Id field populated with a null value
3. See POST to ElasticSearch containing null instead of MapIdPropertyFor Value

Provide ConnectionSettings (if relevant):
nestConnectionSettings.MapIdPropertyFor(x => x.Name);

Provide DebugInformation (if relevant):
bd.Index(b => b
.Index(doc.Index)
.Type(doc.DocType)
.Id(null)
.Document(doc.Document));
Result Id: null

bd.Index(b => b
.Index(doc.Index)
.Type(doc.DocType)
.Document(doc.Document));
Result Id: doc.Name

[Mpdreamz]

Sorry for not getting back to you sooner @spresnal

This is actually by design as per: #2030

For required parameters we revert back to the default but for id which is not we do not because not sending the id is still a valid option e.g elasticsearch will generate the id.