Clone response before put it to the cache (#6932)

`Response.prototype.clone()` must be called before the response used.

This fixes an error from ServiceWorker and failing to load image when the
image is not cached.

app/javascript/mastodon/service_worker/entry.js

@@ -56,10 +56,10 @@ self.addEventListener('fetch', function(event) {
         const fetched = await fetch(event.request);
 
         if (fetched.ok) {
-          await cache.put(event.request.url, fetched);
+          await cache.put(event.request.url, fetched.clone());
         }
 
-        return fetched.clone();
+        return fetched;
       }
 
       return cached;