Fix option to append index.html for `ServeDir` #1083

Job	Run time
check	55s
check-docs	56s
cargo-hack	1m 35s
cargo-public-api-crates	1m 26s
cargo-deny check (advisories)	29s
cargo-deny check (bans licenses sources)	21s
test-msrv	3m 52s
style	5s
test-versions (stable)	1m 8s
test-versions (beta)	1m 20s
test-versions (nightly)	1m 20s
	13m 27s

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