Add comments to handleFetch and only include cookie if it exists

Author: tcompa

src/hooks.server.js

@@ -27,8 +27,18 @@ export async function handle({ event, resolve }) {
 
 /** @type {import('@sveltejs/kit').HandleFetch} */
 export async function handleFetch({ event, request, fetch }) {
-    if (request.url.startsWith(FRACTAL_SERVER_HOST)) {
-        console.log(`Modifying request to ${request.url}, via handleFetch`);
+	/*
+	See:
+	1. https://github.com/fractal-analytics-platform/fractal-web/issues/274
+	2. https://kit.svelte.dev/docs/hooks#server-hooks-handlefetch
+	*/
+	
+    if (
+		request.url.startsWith(FRACTAL_SERVER_HOST) &&
+		'headers' in request &&
+		'cookie' in request.headers
+	) {
+        console.log(`Including cookie into request to ${request.url}, via handleFetch`);
         request.headers.set('cookie', event.request.headers.get('cookie'));
     }
     return fetch(request);