[ServiceWorker] Service Worker doesn't catch favicon requests (#668)

Why doesn't Service Worker catch my favicon request?

```
<link rel="icon" type="image/png" href="/app/favicon-192x192.png" sizes="192x192">
```
**sw.js**
```
onfetch = function(e){
     //nothing happened
     console.log(e.request.url);
};


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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/668