CURLOPT_HEADEROPT(3) on macOS

The text f76431afde31, 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.

set how to send HTTP headers

CURLOPT_HEADEROPT(3) on macOS now