Support for response warning header

[jbudz]

Elasticsearch includes deprecations in a response warning header, elastic/elasticsearch#17804. Including these in the response or log methods may be useful for staying up to date.

[delvedor]

Hello!
You can access the headers when using the callback style, the function should have the following signature: err, body, status, headers.

Currently, I'm working on a complete rewrite of the client, that you can find in the next branch. In the next client, the returned value will be an object with the following keys { body, headers, statusCode, warnings } for both promises and callbacks.

[jbudz]

Awesome, thanks for the info @delvedor