Use close_notify into conduit-lwt-ssl instead a simple shutdown

[dinosaure]

Currently, we don't try to close nicely an SSL connection with conduit-lwt-ssl, see this code:

ocaml-conduit/src/lwt-ssl/conduit_lwt_ssl.ml

Lines 62 to 64 in 66fd34a

	let close socket =
	Lwt_ssl.ssl_shutdown socket >>= fun () ->
	Lwt_ssl.close socket >>= fun () -> Lwt.return_ok ()

#319 highlighted this problem but with #311, we missed this change and we require a release of lwt_ssl/ocaml-ssl. I keep the request as an issue.

[dinosaure]

This issue is lock by ocsigen/lwt_ssl#2