Slice filter: fetch slices in cloned subrequests. Previously, slice subrequest location was selected based on request URI. If request is then redirected to a new location, its context array is cleared, making the slice module loose current slice range information. This lead to broken output. Now subrequests with the NGX_HTTP_SUBREQUEST_CLONE flag are created for slices. Such subrequests stay in the same location as the parent request and keep the right slice context.