Do not cache with Cache API if cache mode is only-if-cached

The previous implementation tried to cache only-if-cached by changing
request mode but it doesn't work because the property is readonly.

However, caching responses cached with HTTP Cache again with Cache API
does not make sense anyway. Just do not cache it, and leave it to HTTP
cache.

app/service-worker.js

@@ -58,6 +58,8 @@ self.addEventListener('fetch', event => {
 	}
 	*/
 
+	if (event.request.cache === 'only-if-cached') return;
+
 	// for everything else, try the network first, falling back to
 	// cache if the user is offline. (If the pages never change, you
 	// might prefer a cache-first approach to a network-first one.)
@@ -66,11 +68,6 @@ self.addEventListener('fetch', event => {
 			.open(`offline${timestamp}`)
 			.then(async cache => {
 				try {
-					if (event.request.cache === 'only-if-cache') {
-						// workaround Chrome devtools bug https://github.com/sveltejs/sapper-template/issues/34
-						event.request.mode = 'same-origin';
-					}
-
 					const response = await fetch(event.request);
 					cache.put(event.request, response.clone());
 					return response;