System proxy settings change ignored by calls to QNetworkAcessManager::get()

When calling QNetworkAccessManager::get() before and after a change of Windows' system proxy settings, the latter call will not account for the change in settings.

Steps to reproduce:

1. Run the attached application.
2. Check the "System proxy" option.
3. Click the "Send request" button. The reply will be valid (assuming your system proxy settings are valid).
4. Change your system proxy settings to be invalid (or valid if they were originally invalid).
5. Click the "Send request" button again. The reply will still be valid (assuming your system proxy settings were originally valid), even though the system proxy settings have changed.

The problem most likely lies below, at line 403 of qnetworkproxy_win.cpp:

void QWindowsSystemProxy::init()
{
    if (initialized)
        return;
    initialized = true;

A workaround is to comment out the first two lines of init().