add unsupported media type response

src/respond.ml

@@ -16,6 +16,7 @@ let bad_request () = empty `Bad_request
 let no_content () = empty `No_content
 let not_modified () = empty `Not_modified
 let not_implemented () = empty `Not_implemented
+let unsupported_media_type () = empty `Unsupported_media_type
 
 let respond_json req jsont status body =
   Logs.info (fun m -> m "%a" pp_status status);