Save a copy of the headers not the original.

Otherwise ExpressJS will complain about trying to modify
headers that have already been sent.

lib/www/server/api/middleware/etag/cache.js

@@ -33,7 +33,7 @@ function saveResponse (res) {
 			const cache = getCache(res);
 			const req = res.req;
 			console.log(`Saving ETag: ${req.method} ${req.url} → ${etag}`);
-			const headers = res.getHeaders();
+			const headers = structuredClone(res.getHeaders());
 			delete headers["set-cookie"];
 			cache[req.url] = {etag, headers};
 		}