Set default proxy scheme in proxy uri #2760

Job	Run time
Test	54s
Test (charset)	1m 2s
Test (cookies)	1m 7s
build_versions (stable)	30s
Test (socks-proxy)	1m 51s
Test	1m 1s
build_versions (beta)	23s
Test (gzip)	1m 13s
Docs	1m 57s
Test (charset)	1m 14s
build_versions (1.71.1)	20s
Test (brotli)	55s
Test (cookies)	2m 27s
Test (json)	1m 7s
Test (socks-proxy)	1m 17s
Lint	1m 1s
Test (native-tls)	1m 8s
cargo-deny (aarch64-apple-ios)	26s
Test (gzip)	55s
cargo-deny (aarch64-linux-android)	19s
Test (brotli)	1m 1s
cargo-deny (i686-pc-windows-gnu)	35s
Test (json)	1m 4s
cargo-deny (i686-pc-windows-msvc)	24s
Test (native-tls)	1m 3s
cargo-deny (i686-unknown-linux-gnu)	22s
Test (rustls-no-provider)	1m 5s
cargo-deny (wasm32-unknown-unknown)	22s
cargo-deny (x86_64-apple-darwin)	20s
cargo-deny (x86_64-apple-ios)	23s
cargo-deny (x86_64-pc-windows-gnu)	26s
cargo-deny (x86_64-pc-windows-msvc)	39s
cargo-deny (x86_64-unknown-linux-gnu)	19s
cargo-deny (x86_64-unknown-redox)	30s
	29m 40s

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