Unify ExplainGet<T> and GetFromUpdate

[Mpdreamz]

Should be generic, and in order to do that in 2.0 UpdateResponse has to be generic as well.

relates to: #1489

[robertlyson]

I really would love to help with this one, but one question pop up. Is it still to be done?

Looks like IUpdateResponse changed a bit between 1.x and 2.0

2.0

public interface IUpdateResponse : IResponse
{
    ShardsMetaData ShardsHit { get; }
    string Index { get; }
    string Type { get; }
    string Id { get; }
    long Version { get; }
}

1.x

public interface IUpdateResponse : IResponse
{
    ShardsMetaData ShardsHit { get; }
    string Index { get; }
    string Type { get; }
    string Id { get; }
    string Version { get; }
    GetFromUpdate Get { get;  }

    T Source<T>() where T : class;
    FieldSelection<T> Fields<T>() where T : class;
}

[Mpdreamz]

Yeah the added properties and methods were introduced in the linked issue:

https://github.com/elastic/elasticsearch-net/pull/1489/files

They were introduced as methods because making the response object generic would be a breaking change :(

[robertlyson]

Thank you @Mpdreamz, now I understand :)
I'll do my best to handle this.

Also I'll try to write tests #1481 way :)

[Mpdreamz]

@gmarz @russcam would love to see this in alpha1

See: #1296

[robertlyson]

@Mpdreamz thanks for handling 👍. 👎 for @me.