Use self.integrity_uri over self.hexdigest_integrity_uri(digest), as the
latter always returned nil leading to improper cache invalidation.
We are alos enabling caching of `tmp/cache/assets/sprockets` again, to
get those performance gains.
However we still want to make ClassMethods work!
And we'll need to fix codes using the wrong include/prepend
along the way. There were a lot of misuses.