fix: Overriden custom cookies used inside withBlitzAuth (#4424)

* fix: use append rather than set

* Create ninety-stingrays-double.md

Author: siddhsuresh

.changeset/ninety-stingrays-double.md

@@ -0,0 +1,5 @@
+---
+"@blitzjs/auth": patch
+---
+
+fix: Overriden custom cookies used inside `withBlitzAuth`

packages/blitz-auth/src/server/auth-sessions.ts

@@ -466,7 +466,7 @@ export class SessionContextClass implements SessionContext {
     }
     const cookieHeaders = this._headers.get("set-cookie")
     if (response instanceof Response) {
-      response.headers.set("Set-Cookie", cookieHeaders!)
+      response.headers.append("Set-Cookie", cookieHeaders!)
     } else {
       response.setHeader("Set-Cookie", splitCookiesString(cookieHeaders!))
     }