CURLINFO_HEADER_SIZE(3) on macOS

The text 1ec58092414a, 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.

get size of retrieved headers

CURLINFO_HEADER_SIZE(3) on macOS now