Please, consider applying
honor system proxy
Have you tested this when no ENV['http_proxy'] is set? I think in current form it will crash on undefined.
Can you wrap it in a test for it being set, if not then don't set proxy.
This is not needed. In case you mention the net result is passing proxy: undefined, which is effectively the same as not specifying proxy key at all.
I have tested, and it works ok.
Ah, doh. ok :)