Merge branch 'link2xt/redirect-follow' into 'master'

Follow redirects with `fetch`

Closes #85

See merge request rysiekpl/libresilient!29

service-worker.js

@@ -1012,7 +1012,23 @@ let initFromRequest = (req) => {
         //mode: req.mode,        TODO: commented out because mode: navigate is haram in service worker, it seems
         //credentials: req.credentials, TODO: commented out because credentials: "include" is haram if the Access-Control-Allow-Origin header is '*'
         cache: req.cache,
-        redirect: req.redirect,
+
+        // `req.redirect` is usually "manual":
+        // <https://stackoverflow.com/questions/42716082/fetch-api-whats-the-use-of-redirect-manual>
+        //
+        // It is not useful for fetch() requests
+        // as on redirect it returns a response with "opaqueredirect" type
+        // 0 response code and no way to get actual location:
+        // <https://stackoverflow.com/questions/39735496/redirect-after-a-fetch-post-call>.
+        //
+        // This approach has a drawback of not changing the URL
+        // in a browser when following the redirect for the page
+        // (in contrast to some resource like an image where it does not matter),
+        // but there is no better option until browsers provide some way
+        // to access redirect responses such as the one
+        // proposed in <https://github.com/whatwg/fetch/issues/601>.
+        redirect: "follow",
+
         referrer: req.referrer,
         integrity: req.integrity
     }