Return value of the id column is nil all the time

If the id column is present in the _source attributes it is not correctly mapped to the result.id because Hashie::Mash explicitly defines `id` and `type` columns [here](https://github.com/intridea/hashie/blob/master/lib/hashie/mash.rb#L73-L79)
So I have added an explicit mapping of the id column to `record._source` if it responds to it.