Range filter: allowed ranges on empty files (ticket #1031). As per RFC 2616 / RFC 7233, any range request to an empty file is expected to result in 416 Range Not Satisfiable response, as there cannot be a "byte-range-spec whose first-byte-pos is less than the current length of the entity-body". On the other hand, this makes use of byte-range requests inconvenient in some cases, as reported for the slice module here: http://mailman.nginx.org/pipermail/nginx-devel/2017-June/010177.html This commit changes range filter to instead return 200 if the file is empty and the range requested starts at 0.