[6.2] Always include the _index and _id for nested search hits

[codebrain]

https://www.elastic.co/guide/en/elasticsearch/reference/current/release-notes-6.2.0.html

Aggregations: Always include the _index and _id for nested search hits - elastic/elasticsearch#27201

[codebrain]

I believe these are already implemented:

• elasticsearch-net/src/Nest/Search/Search/Hits/Hit.cs

  Line 64 in e222391

  public string Index { get; internal set; }

• elasticsearch-net/src/Nest/Search/Search/Hits/Hit.cs

  Line 81 in e222391

  public string Id { get; internal set; }

Thoughts? @russcam

[codebrain]

Have added check to integration test in master, no action needed in 6.x.

5dc01c4