Replaced the usage of undocumented res._header with `res.headersSen…

…t` (regression-safe)

- https://github.com/nodejs/node/blob/93e0bf9abf350637d772bcce14a5d9527b733300/lib/_http_outgoing.js#L741-L746

index.js

@@ -272,7 +272,7 @@ function session(options) {
           return ret;
         }
 
-        if (!res._header) {
+        if (typeof res.headersSent === 'boolean' ? !res.headersSent : !res._header) {
           res.writeHead(res.statusCode)
         }