[REG 5.15.1 -> 5.15.2] firstPartyUrl for cookie filter has subdomains stripped

Starting with Qt 5.15.2, the firstPartyUrl in QWebEngineCookieStore::FilterRequest looks like https://example.com/, even if the cookie is actually set on e.g. https://www.example.com/.

This can be seen by patching simplebrowser to log those URLs, by adding this in main:

QWebEngineProfile::defaultProfile()->cookieStore()->setCookieFilter([](auto request) {
    qDebug() << request.firstPartyUrl;
    return true;
});

And then e.g. visiting mouser.com. With Qt 5.15.1, the URLs logged are https://eu.mouser.com/ and https://www.mouser.com/, while with Qt 5.15.2 they all are https://mouser.com/.

I'm not sure if this was a deliberate change or a bug - but if deliberate, it seems like quite a breaking change for a patch release. Note that there's a privacy impact as well, as this could lead to cookies being blocked by the user to be accepted again (as the filter isn't working as intended).