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.