Proxy helper should strip http:// before use if it exists

[jblaine]

It's standard to define proxies in chef config in URL form. Cygwin doesn't like that. The fix below to the single line of code solves the issue and retains backward compatibility.

module Cygwin
  module Helpers
    def proxy_command
      Chef::Config['http_proxy'].nil? ? "" : "--proxy #{Chef::Config['http_proxy'].gsub('http://', '')}"
    end
  end
end