guzzlehttp/guzzle: Silent HTTPS-Proxy Downgrade to Cleartext

Impact The built-in cURL handlers GuzzleHttp\Handler\CurlHandler and GuzzleHttp\Handler\CurlMultiHandler, used by default whenever the PHP cURL extension is available accept an https:// proxy — a proxy reached over a TLS-encrypted connection — through the proxy request option, client-level proxy...