Nest 2.0.0 Indexing id-less object incorrectly selects PUT method, should be POST

[wewebber]

If I do (in F#):

type DocType = { Id : string ; Name : string }
let myDoc = { Id = null; Name = "foo" }
client.Index(IndexRequest(DocumentPath(myDoc)))

then the request that gets generated is

PUT /doctype/doctype/
{ body }

which causes an "Invalid Request" error from ElasticSearch (2.1.1). The request should be:

POST /doctype/doctype/
{ body }

The problematic code is at line 20 of https://github.com/elastic/elasticsearch-net/blob/82c938893b2ff4ddca03a8e977ad14a16da712ba/src/Nest/Document/Single/Index/IndexRequest.cs

    protected override HttpMethod HttpMethod => ((IIndexRequest<TDocument>)this).Id 
                 == null ? HttpMethod.POST : HttpMethod.PUT;

The Id property here is Nest.Id object wrapped around the document itself. Its GetString() method will return null, but the object itself is not null.