Update to Cohttp.0.12.0 API

This is the API in [mirage/ocaml-cohttp#196]

Signed-off-by: David Scott <dave.scott@eu.citrix.com>

src/impl.ml

@@ -689,14 +689,14 @@ let write_stream common s destination source_protocol destination_protocol preze
         | Some x ->
           begin match Re_str.bounded_split_delim (Re_str.regexp_string ":") x 2 with
           | [ user; pass ] ->
-            let b = Cohttp.Auth.string_of_resp (`Basic (user, pass)) in
+            let b = Cohttp.Auth.string_of_credential (`Basic (user, pass)) in
             Header.add headers "authorization" b
           | _ ->
             Printf.fprintf stderr "I don't know how to handle authentication for this URI.\n Try scheme://user:password@host/path\n";
             exit 1
           end in
       let request = Cohttp.Request.make ~meth:`PUT ~version:`HTTP_1_1 ~headers uri' in
-      Request.write (fun t _ -> return ()) request c >>= fun () ->
+      Request.write (fun _ -> return ()) request c >>= fun () ->
       Response.read (Cohttp_unbuffered_io.make_input c) >>= fun r ->
       begin match r with
       | `Invalid x -> fail (Failure (Printf.sprintf "Invalid HTTP response: %s"