MoreLikeThis query should accept documents or ids

[robertlyson]

Hello guys,

Right now we are unable to run query like this one:

var searchResponse = client.Search<Document>(s => s.Query(q => q
    .MoreLikeThis(mlt => mlt.Documents(docs => docs.Document(new Document {Id = 1})))));

It sends empty query to elasticsearch:

StatusCode: 200, 
Method: POST, 
Url: http://localhost:9200/my_index/document/_search, 
Request: {}, 

Documentation stays that

The only required parameters are either docs, ids or like_text

so I think this PR should resolve the issue.

Reported on SO.

Thanks.

[gmarz]

LGTM 👍 thank you @robertlyson !