Add support for "Expect: 100-continue" header #1853

Job	Run time
Docs	42s
build_versions (stable)	19s
cargo-deny (aarch64-apple-ios)	22s
build_versions (beta)	16s
cargo-deny (aarch64-linux-android)	20s
Test	43s
build_versions (1.61.0)	17s
Lint	11s
cargo-deny (i686-pc-windows-gnu)	22s
Test (json)	45s
cargo-deny (i686-pc-windows-msvc)	18s
Test (charset)	44s
Test (cookies)	47s
cargo-deny (i686-unknown-linux-gnu)	22s
Test (socks-proxy)	9s
cargo-deny (wasm32-unknown-unknown)	19s
Test (native-certs)	44s
cargo-deny (x86_64-apple-darwin)	17s
Test (gzip)	44s
cargo-deny (x86_64-apple-ios)	22s
cargo-deny (x86_64-pc-windows-gnu)	23s
Test (brotli)	46s
cargo-deny (x86_64-pc-windows-msvc)	19s
Test (http-crate)	47s
Test (http-interop)	46s
cargo-deny (x86_64-unknown-linux-gnu)	21s
Test (http-crate http-interop)	45s
cargo-deny (x86_64-unknown-redox)	20s
Test (tls)	6s
Test (tls, json)	2s
Test (tls, charset)	4s
Test (tls, cookies)	5s
Test (tls, socks-proxy)	2s
Test (tls, native-certs)	2s
Test (tls, gzip)	6s
Test (tls, brotli)	6s
Test (tls, http-crate)	3s
Test (tls, http-interop)	4s
Test (tls, http-crate http-interop)	4s
Test (native-tls)	48s
Test (native-tls, json)	49s
Test (native-tls, charset)	46s
Test (native-tls, cookies)	53s
Test (native-tls, socks-proxy)	9s
Test (native-tls, native-certs)	50s
Test (native-tls, gzip)	41s
Test (native-tls, brotli)	47s
Test (native-tls, http-crate)	53s
Test (native-tls, http-interop)	52s
Test (native-tls, http-crate http-interop)	51s
	22m 33s

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