MINOR: hlua: Use the buffer instead of the HTTP message to get HTTP headers

MINOR: hlua: Use the buffer instead of the HTTP message to get HTTP headers

hlua_http_get_headers() function was using the HTTP message from the stream
TXN to retrieve headers from a message. However, this will be an issue to
update the lua HTTP applet to use its own buffers. Indeed, in that case,
information from the channels will be unavailable. So now,
hlua_http_get_headers() is now using a buffer containing an HTX message. It
is just an API change bacause, internally, the function was already
manipulation an HTX message.