cohttp-eio(client): move Host header to the font

CHANGES.md

@@ -1,4 +1,5 @@
 ## Unreleased
+# cohttp-eio: ensure "Host" header is the first header in http client requests (bikallem #939)
 - cohttp-eio: add Content-Length header to request/response (bikallem #929)
 - cohttp-eio: add cohttp-eio client api - Cohttp_eio.Client (bikallem #879)
 - http: add requires_content_length function for requests and responses (bikallem #879)

cohttp-eio/src/client.ml

@@ -72,15 +72,16 @@ let response buf_read =
 
 let call ?meth ?version ?(headers = Http.Header.init ()) ?(body = Body.Empty)
     ~conn host resource_path =
+  let host_hdr = "Host" in
   let headers =
-    if not (Http.Header.mem headers "Host") then
+    if not (Http.Header.mem headers host_hdr) then
       let host =
         match host with
         | host, Some port -> host ^ ":" ^ string_of_int port
         | host, None -> host
       in
-      Http.Header.add headers "Host" host
-    else headers
+      Http.Header.add headers host_hdr host
+    else Http.Header.move_to_front headers host_hdr
   in
   let request = Http.Request.make ?meth ?version ~headers resource_path in
   Buf_write.with_flow ~initial_size:0x1000 conn (fun writer ->