Nar listings (.ls) missing Content-Encoding header

[bennofs]

This used to work, but some change recently means that NAR listings (.ls) files no longer have a Content-Encoding header, even though they are brotli-compressed:

$ http GET http://cache.nixos.org/xgzxjzg4bz9b6k1vypxwk7zkvhwybdxq.ls
HTTP/1.1 200 OK
Accept-Ranges: bytes
Connection: keep-alive
Content-Length: 106
Content-Type: application/json
Date: Mon, 06 Aug 2018 08:32:07 GMT
ETag: "43badaac32089172cdd4f7ef2d711afb"
Last-Modified: Thu, 02 Aug 2018 18:52:38 GMT
Server: AmazonS3
Via: 1.1 261d871caba4097bc29b3ff8bd23af86.cloudfront.net (CloudFront)
X-Amz-Cf-Id: w3JDAyQNioO44wc4ePxjmYmkz9Gm9C2y-0YwuD8GXuve8tQbWPyn3Q==
X-Cache: Miss from cloudfront

+-----------------------------------------+
| NOTE: binary data not shown in terminal |
+-----------------------------------------+

This is an issue in nix-community/nix-index#24, as nix-index is no longer able to correctly decode the response (it assumes plain text if there is no Content-Encoding header).

[bennofs]

I did a quick rough bisection, it appears that somewhere between builds https://hydra.nixos.org/build/78901776 (no content-encoding header) and https://hydra.nixos.org/build/78862369#tabs-summary (has content-encoding header) something was broken. Has anything special happened from 2018/08/01 to 2018/08/02?