Fix 3rd party login issue on macOS (#4739)

* Fix use-after-free error

* Ignore http OPTIONS method

* Make sure response has sent before closing the http server

src/slic3r/GUI/HttpServer.hpp

@@ -28,6 +28,7 @@ class http_headers
 
     std::map<std::string, std::string> headers;
 
+    friend class session;
 public:
     std::string get_url() { return url; }