CURLOPT_PROXYHEADER(3) on macOS

The text 75ad82cbf8c1, as macOS 10.12.5, macOS 10.12.4, macOS 10.12.3, macOS 10.12.2, macOS 10.12.1, macOS 10.12 shipped it.

custom HTTP headers to pass to proxy

CURLOPT_PROXYHEADER(3) on macOS now