find_cert_bundle returns NULL on Linux

[cschroedl-gov]

R version: 3.3
httr version: 1.2.1
OS: opensuse Leap 42.2

The function find_cert_bundle currently returns NULL on Linux. The first condition in the function indicates that we should return early if we're not on Windows. However, the rest of the function /looks/ like it is consulting *nix system resources. Looks can be deceiving, so I'm curious what you think :)

[hadley]

This is by design as only on windows is the bundled cert needed.