add support for the NO_PROXY environment variable when fetching isos.

uri-open by default uses a proxy for each request if the corresponding *_proxy env variable is set.
This commit makes open-uri ignore those variables on a by request basis if the target hostname is found in the NO_PROXY environment variable.
This allows veewee to fetch external resources like the VirtualBox additions via proxy and locally available isos directly.
Merge pull request #561 from databus23/no_proxy_env

add support for the NO_PROXY environment variable when fetching isos.