fixed cookie refresh bug

Issue maxcountryman#824. Before, if a user was logged in with the login_user function when the remember parameter was set to false, their cookies would still be refreshed if the "REMEMBER_COOKIE_REFRESH_EACH_REQUEST" configuration option was set to true. This happens because if the login_user function has the remember parameter be false, it doesn't assign session["_rememeber"] any value. When session["_rememeber"] doesn't have any value and the "REMEMBER_COOKIE_REFRESH_EACH_REQUEST" configuration option is set to true, the _update_remember_cookie function sets the session["_rememeber"] value to "set". This fix makes it so if the login_user function is given false for the remember parameter, instead of leaving session["_remember"] empty, it  sets the value to "unset".

src/flask_login/login_manager.py

@@ -402,6 +402,8 @@ def _update_remember_cookie(self, response):
                 self._set_cookie(response)
             elif operation == "clear":
                 self._clear_cookie(response)
+            elif operation == "unset":
+                session["_remember"] = "unset"
 
         return response
 

src/flask_login/utils.py

@@ -198,6 +198,8 @@ def login_user(user, remember=False, duration=None, force=False, fresh=True):
                 raise Exception(
                     f"duration must be a datetime.timedelta, instead got: {duration}"
                 ) from e
+    else:
+        session["_remember"] = "unset"
 
     current_app.login_manager._update_request_context_with_user(user)
     user_logged_in.send(current_app._get_current_object(), user=_get_user())