Set default proxy scheme in proxy uri #2761

Job	Run time
Lint	1m 5s
Test	1m 10s
build_versions (stable)	20s
Test (charset)	56s
Test	43s
build_versions (beta)	18s
cargo-deny (aarch64-apple-ios)	20s
Test (cookies)	1m 30s
Test (charset)	1m 6s
build_versions (1.71.1)	18s
cargo-deny (aarch64-linux-android)	25s
Test (socks-proxy)	51s
Test (cookies)	1m 44s
cargo-deny (i686-pc-windows-gnu)	21s
Test (socks-proxy)	1m 4s
Test (gzip)	1m 6s
cargo-deny (i686-pc-windows-msvc)	24s
Test (gzip)	57s
Docs	1m 53s
Test (brotli)	1m 11s
cargo-deny (i686-unknown-linux-gnu)	22s
Test (brotli)	1m 7s
Test (json)	1m 4s
Test (json)	1m 4s
cargo-deny (wasm32-unknown-unknown)	26s
Test (native-tls)	1m 7s
cargo-deny (x86_64-apple-darwin)	23s
Test (native-tls)	1m 4s
cargo-deny (x86_64-apple-ios)	33s
Test (rustls-no-provider)	1m 6s
cargo-deny (x86_64-pc-windows-gnu)	32s
cargo-deny (x86_64-pc-windows-msvc)	20s
cargo-deny (x86_64-unknown-linux-gnu)	24s
cargo-deny (x86_64-unknown-redox)	23s
	27m 37s

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