Vary accept-encoding header not always set for compressible content

[kanongil]

Currently the accept-encoding header is only added to the vary header when the response contains a content-encoding. This works but can cause intermediate caches to only return the non-encoded response when an initial request does not contain a compatible content-encoding, causing compatible clients to receive non-encoded content.

To fix this, accept-encoding should be added to the vary header to all payload containing responses with compressible content.