copying with `replace: 'all'` seems to be the only option which works

I find that the default usage `assets()` does not copy the content of my assets directory, nor does `assets({replace: 'old'})` because it checks the date of the destination directory and finds it is newer than the source. i.e. It doesn't check the contents of the directory.

The only case which works for me is `assets({replace: 'all'})`.