cors: Don't overwrite vary header set by the inner service #1063

Job	Run time
check	49s
cargo-public-api-crates	3m 4s
check-docs	49s
cargo-hack	1m 32s
cargo-deny check (advisories)	19s
cargo-deny check (bans licenses sources)	22s
style	6s
test-versions (stable)	1m 46s
test-msrv	2m 56s
test-versions (beta)	1m 37s
test-versions (nightly)	1m 34s
	14m 54s

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