MINOR: http: add the new sample fetches req.hdr_names and res.hdr_names

MINOR: http: add the new sample fetches req.hdr_names and res.hdr_names

These new sample fetches retrieve the list of header names as they appear
in the request or response. This can be used for debugging, for statistics
as well as an aid to better detect the presence of proxies or plugins on
some browsers, which alter the request compared to a regular browser by
adding or reordering headers.