pub fn merge_vary(headers: &mut HeaderMap, token: &str)Expand description
Merge one token into a response’s Vary header (DW-027): appended to
the existing comma-separated list when not already present
(case-insensitive), created otherwise. RFC 9110 permits MULTIPLE
Vary field lines; every line is folded into the membership check
and the merged value, and the result replaces the set with one line —
reading only the first line would drop the later lines’ tokens and
corrupt cache keys for upstreams that split their Vary. Correct
Vary is what keeps a shared cache from serving a compressed (or
origin-specific) response to a client that could not have received
it.