Don't export Header struct #1716

Job	Run time
Lint	21s
Docs	28s
Test	8s
Test (json)	28s
Test (charset)	25s
Test (cookies)	27s
cargo-deny (aarch64-apple-ios)	22s
Test (socks-proxy)	1s
Test (native-certs)	6s
cargo-deny (aarch64-linux-android)	20s
Test (gzip)	27s
cargo-deny (i686-pc-windows-gnu)	17s
cargo-deny (i686-pc-windows-msvc)	23s
Test (brotli)	28s
cargo-deny (i686-unknown-linux-gnu)	19s
Test (http-interop)	27s
cargo-deny (wasm32-unknown-unknown)	22s
Test (tls)	1s
cargo-deny (x86_64-apple-darwin)	18s
Test (tls, json)	1s
cargo-deny (x86_64-apple-ios)	21s
Test (tls, charset)	1s
cargo-deny (x86_64-pc-windows-gnu)	20s
Test (tls, cookies)	1s
cargo-deny (x86_64-pc-windows-msvc)	21s
cargo-deny (x86_64-unknown-linux-gnu)	21s
Test (tls, socks-proxy)	1s
cargo-deny (x86_64-unknown-redox)	20s
Test (tls, native-certs)	1s
Test (tls, gzip)	1s
Test (tls, brotli)	1s
Test (tls, http-interop)	1s
Test (native-tls)	5s
Test (native-tls, json)	1s
Test (native-tls, charset)	2s
Test (native-tls, cookies)	5s
Test (native-tls, socks-proxy)	1s
Test (native-tls, native-certs)	1s
Test (native-tls, gzip)	2s
Test (native-tls, brotli)	5s
Test (native-tls, http-interop)	1s
	8m 22s

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