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.

src/http/modules/ngx_http_slice_filter_module.c

diff --git a/src/http/modules/ngx_http_slice_filter_module.c b/src/http/modules/ngx_http_slice_filter_module.c
index 5e149b4..2005939 100644
--- a/src/http/modules/ngx_http_slice_filter_module.c
+++ b/src/http/modules/ngx_http_slice_filter_module.c
@@ -244,7 +244,10 @@
         return rc;
     }
 
-    if (ngx_http_subrequest(r, &r->uri, &r->args, &sr, NULL, 0) != NGX_OK) {
+    if (ngx_http_subrequest(r, &r->uri, &r->args, &sr, NULL,
+                            NGX_HTTP_SUBREQUEST_CLONE)
+        != NGX_OK)
+    {
         return NGX_ERROR;
     }