MINOR: httpclient/lua: implement the headers in the response object

MINOR: httpclient/lua: implement the headers in the response object

Provide a new field "headers" in the response of the HTTPClient, which
contains all headers of the response.

This field is a multi-dimensionnal table which could be represented this
way in lua:

    headers = {
       ["content-type"] = { "text/html" },
       ["cache-control"] = { "no-cache" }
    }