Use more constants for headers #2750

Job	Run time
build_versions (stable)	8s
Test	53s
build_versions (beta)	14s
cargo-deny (aarch64-apple-ios)	18s
Test (charset)	56s
Test	48s
build_versions (1.71.1)	16s
cargo-deny (aarch64-linux-android)	22s
Test (cookies)	59s
Test (charset)	47s
cargo-deny (i686-pc-windows-gnu)	21s
Test (socks-proxy)	59s
Test (cookies)	48s
cargo-deny (i686-pc-windows-msvc)	23s
Docs	1m 45s
Test (gzip)	55s
Test (socks-proxy)	45s
cargo-deny (i686-unknown-linux-gnu)	18s
Test (brotli)	58s
Test (gzip)	45s
cargo-deny (wasm32-unknown-unknown)	23s
Test (json)	57s
Lint	36s
Test (brotli)	48s
cargo-deny (x86_64-apple-darwin)	23s
Test (native-tls)	58s
Test (json)	49s
cargo-deny (x86_64-apple-ios)	20s
Test (native-tls)	48s
cargo-deny (x86_64-pc-windows-gnu)	19s
Test (rustls-no-provider)	48s
cargo-deny (x86_64-pc-windows-msvc)	21s
cargo-deny (x86_64-unknown-linux-gnu)	21s
cargo-deny (x86_64-unknown-redox)	17s
	21m 46s

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