fix (#229): remove header mutation

On IOS, mutate an HTTP response would throw with the error "Headers object's guard is 'immutable". The idea of that mutation was to provide a way in frontend land to detect when a response is fresh or stale from a cache. As of today, we don't use that information so we removed the associated code

client/worker/sw_cache.js

@@ -97,7 +97,6 @@ function cacheFirstStrategy(event){
             }
 
             fetchAndCache(event).catch(nil);
-            response.headers.append("Content-Stale", "yes");
             return response;
         });
     });