CURLOPT_KEEP_SENDING_ON_ERROR(3) on macOS

The text bcc113ad5d24, as macOS 10.12.5, macOS 10.12.4, macOS 10.12.3, macOS 10.12.2 shipped it.

keep sending on early HTTP response >= 300

CURLOPT_KEEP_SENDING_ON_ERROR(3) on macOS now