Fix content length for HEAD requests

According to RFCs, HEAD requests do not have any content, but may include
content length to inform the client about the size of the resource.

closes inhabitedtype#87

lib/response.ml

@@ -57,6 +57,14 @@ let server_error = `Error `Internal_server_error
 let body_length ?(proxy=false) ~request_method { status; headers; _ } =
   match status, request_method with
   | (`No_content | `Not_modified), _           -> `Fixed 0L
+  | _, `HEAD                                   ->
+    (* From RFC7230§3.3.2:
+         A server MAY send a Content-Length header field in a response to a
+         HEAD request (Section 4.3.2 of [RFC7231]); a server MUST NOT send
+         Content-Length in such a response unless its field-value equals the
+         decimal number of octets that would have been sent in the payload body
+         of a response if the same request had used the GET method. *)
+    `Fixed 0L
   | s, _        when Status.is_informational s -> `Fixed 0L
   | s, `CONNECT when Status.is_successful s    -> `Close_delimited
   | _, _                                       ->

lib_test/test_client_connection.ml

@@ -207,6 +207,31 @@ let test_get () =
   read_string t "d\r\nHello, world!\r\n0\r\n\r\n";
 ;;
 
+let test_head () =
+  let error_handler_called = ref false in
+  let request' = Request.create `HEAD "/" in
+  let response = Response.create
+      ~headers:(Headers.of_list ["content-length", "5"])
+      `OK
+  in
+
+  let t = create ?config:None in
+  let body =
+    request
+      t
+      request'
+      ~response_handler:(default_response_handler response)
+      ~error_handler:(fun _ -> error_handler_called := true)
+  in
+  Body.close_writer body;
+  write_request t request';
+  read_response t response;
+  let c = read_eof t Bigstringaf.empty ~off:0 ~len:0 in
+  Alcotest.(check int) "read_eof with no input returns 0" 0 c;
+  connection_is_shutdown t;
+  Alcotest.(check bool) "error handler not called" false !error_handler_called;
+;;
+
 let test_get_last_close () =
   (* Multiple GET requests, the last one closes the connection *)
   let request' = Request.create `GET "/" in
@@ -1355,6 +1380,7 @@ let test_chunked_error () =
 let tests =
   [ "commit parse after every header line", `Quick, test_commit_parse_after_every_header
   ; "GET"         , `Quick, test_get
+  ; "HEAD"        , `Quick, test_head
   ; "Response EOF", `Quick, test_response_eof
   ; "Response header order preserved", `Quick, test_response_header_order
   ; "report_exn"  , `Quick, test_report_exn