Do not systematically create a XMLHttpRequest object.

	Once linked, the library would always create a XMLHttpRequest
        on boot to check for supported features. This would force a
        runtime dependency on XHR even if the feature is not actually
        used.

        A problematic exemple would be running some library unit tests
        with node, which does not include XHR by default. Loading the
        library will fail right upon importing it, no matter whether
        we actually use requests or not.

        This patch make this feature check lazy so that we only fail
        if we actually try to perform a request.

cohttp-lwt-jsoo/src/cohttp_lwt_jsoo.ml

@@ -31,9 +31,10 @@ end
 
 let xhr_response_supported =
   (* from http://stackoverflow.com/questions/8926505/how-to-feature-detect-if-xmlhttprequest-supports-responsetype-arraybuffer *)
-  let xhr = XmlHttpRequest.create () in
-  let rt = xhr##.responseType in
-  Js.to_string (Js.typeof rt) = "string"
+  lazy
+    (let xhr = XmlHttpRequest.create () in
+     let rt = xhr##.responseType in
+     Js.to_string (Js.typeof rt) = "string")
 
 let binary_string str =
   let len = String.length str in
@@ -111,7 +112,7 @@ module Body_builder (P : Params) = struct
           (fun () -> `String (Js.string ""))
           (fun s -> `String s)
       in
-      match xhr_response_supported with
+      match Lazy.force xhr_response_supported with
       | true when Js.Opt.return xml##.response == Js.null ->
           Log.warn (fun m -> m "XHR Response is null; using empty string");
           `String (Js.string "")
@@ -195,7 +196,8 @@ module Make_client_async (P : Params) = Make_api (struct
   let call ?headers ?body meth uri =
     let xml = XmlHttpRequest.create () in
     xml##.withCredentials := Js.bool P.with_credentials;
-    if xhr_response_supported then xml##.responseType := Js.string "arraybuffer";
+    if Lazy.force xhr_response_supported then
+      xml##.responseType := Js.string "arraybuffer";
     let (res : (Http.Response.t Lwt.t * CLB.t) Lwt.t), wake = Lwt.task () in
     let () =
       xml
@@ -283,7 +285,8 @@ module Make_client_sync (P : Params) = Make_api (struct
   let call ?headers ?body meth uri =
     let xml = XmlHttpRequest.create () in
     xml##.withCredentials := Js.bool P.with_credentials;
-    if xhr_response_supported then xml##.responseType := Js.string "arraybuffer";
+    if Lazy.force xhr_response_supported then
+      xml##.responseType := Js.string "arraybuffer";
     let () =
       xml
       ## (_open