Use header constants #2371

Job	Run time
Docs	29s
Lint	21s
build_versions (stable)	16s
Test	1m 2s
build_versions (beta)	18s
Test	35s
build_versions (1.67.0)	20s
Test (charset)	43s
cargo-deny (aarch64-apple-ios)	20s
Test (charset)	32s
Test (cookies)	59s
cargo-deny (aarch64-linux-android)	26s
Test (cookies)	39s
Test (socks-proxy)	52s
cargo-deny (i686-pc-windows-gnu)	21s
Test (socks-proxy)	32s
Test (gzip)	53s
cargo-deny (i686-pc-windows-msvc)	21s
Test (gzip)	33s
Test (brotli)	42s
Test (brotli)	34s
Test (json)	45s
cargo-deny (i686-unknown-linux-gnu)	19s
cargo-deny (wasm32-unknown-unknown)	19s
Test (native-tls)	1m 0s
Test (json)	33s
cargo-deny (x86_64-apple-darwin)	23s
Test (native-tls)	42s
cargo-deny (x86_64-apple-ios)	27s
cargo-deny (x86_64-pc-windows-gnu)	17s
cargo-deny (x86_64-pc-windows-msvc)	20s
cargo-deny (x86_64-unknown-linux-gnu)	21s
cargo-deny (x86_64-unknown-redox)	26s
	17m 40s

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