Add support for "Expect: 100-continue" header #1701

Job	Run time
Lint	12s
Test	46s
cargo-deny (aarch64-apple-ios)	23s
Test (json)	1m 24s
cargo-deny (aarch64-linux-android)	21s
Docs	22s
Test (charset)	2m 51s
cargo-deny (i686-pc-windows-gnu)	18s
cargo-deny (i686-pc-windows-msvc)	19s
Test (cookies)	41s
cargo-deny (i686-unknown-linux-gnu)	30s
Test (socks-proxy)	38s
cargo-deny (wasm32-unknown-unknown)	25s
Test (native-certs)	44s
cargo-deny (x86_64-apple-darwin)	20s
Test (gzip)	39s
cargo-deny (x86_64-apple-ios)	24s
Test (brotli)	1m 6s
cargo-deny (x86_64-pc-windows-gnu)	27s
Test (http-interop)	1m 28s
cargo-deny (x86_64-pc-windows-msvc)	22s
Test (tls)	38s
cargo-deny (x86_64-unknown-linux-gnu)	28s
Test (tls, json)	44s
cargo-deny (x86_64-unknown-redox)	24s
Test (tls, charset)	43s
Test (tls, cookies)	46s
Test (tls, socks-proxy)	1m 31s
Test (tls, native-certs)	1m 16s
Test (tls, gzip)	40s
Test (tls, brotli)	41s
Test (tls, http-interop)	43s
Test (native-tls)	1m 19s
Test (native-tls, json)	54s
Test (native-tls, charset)	50s
Test (native-tls, cookies)	45s
Test (native-tls, socks-proxy)	40s
Test (native-tls, native-certs)	1m 36s
Test (native-tls, gzip)	41s
Test (native-tls, brotli)	42s
Test (native-tls, http-interop)	47s
	31m 28s

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