Add source to inner_hits

[picandocodigo]

Transferred from elastic/elasticsearch-ruby#1083, original PR by @eliskah

Original Pull Request message

This PR should fix following:

When using field collapsing, it is possible to define _source for inner hits, like this:

{
  "collapse": {
    "field" : "user_id",
    "inner_hits" : {
      "name" : "records",
      "size" : 5,
      "_source" : ["content", "updated_at"]
    }
  },
  "_source" : false,
  "size" : 10
}

DSL supports collapsing with inner hits fine, but if I try to define the _source inside the inner_hits block, it's used in the top level _source instead. See this example:

require 'json'
require 'elasticsearch/dsl'

include Elasticsearch::DSL

definition = search do
  collapse :user_id do
    inner_hits :records do
      size 5
      _source ["content", "updated_at"]
    end
  end
  _source false
  size 10
end

puts definition.to_hash.to_json

Returns:

{
  "size": 10,
  "collapse": {
    "field": "user_id",
    "inner_hits": {
      "name": "records",
      "size": 5
    }
  },
  "_source": [
    "content",
    "updated_at"
  ]
}

I would expect to get the query as above: With top level _source value of false and inner hits level _source with the fields "content" and "updated_at".

[h6ah4i]

+1