Add support for "Expect: 100-continue" header #1703

Job	Run time
Lint	20s
cargo-deny (aarch64-apple-ios)	30s
Test	1m 2s
Docs	44s
cargo-deny (aarch64-linux-android)	23s
Test (json)	1m 7s
cargo-deny (i686-pc-windows-gnu)	29s
Test (charset)	1m 10s
cargo-deny (i686-pc-windows-msvc)	19s
Test (cookies)	47s
cargo-deny (i686-unknown-linux-gnu)	19s
Test (socks-proxy)	41s
cargo-deny (wasm32-unknown-unknown)	25s
Test (native-certs)	47s
cargo-deny (x86_64-apple-darwin)	26s
Test (gzip)	43s
cargo-deny (x86_64-apple-ios)	21s
Test (brotli)	40s
cargo-deny (x86_64-pc-windows-gnu)	38s
Test (http-interop)	40s
cargo-deny (x86_64-pc-windows-msvc)	24s
Test (tls)	1m 18s
cargo-deny (x86_64-unknown-linux-gnu)	20s
Test (tls, json)	43s
cargo-deny (x86_64-unknown-redox)	25s
Test (tls, charset)	1m 33s
Test (tls, cookies)	1m 32s
Test (tls, socks-proxy)	1m 5s
Test (tls, native-certs)	42s
Test (tls, gzip)	58s
Test (tls, brotli)	43s
Test (tls, http-interop)	45s
Test (native-tls)	41s
Test (native-tls, json)	42s
Test (native-tls, charset)	41s
Test (native-tls, cookies)	1m 40s
Test (native-tls, socks-proxy)	42s
Test (native-tls, native-certs)	40s
Test (native-tls, gzip)	46s
Test (native-tls, brotli)	50s
Test (native-tls, http-interop)	1m 28s
	31m 9s

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