The `shouldBypassProxy` method from `ProxyUtils.swift` did not account
for `NO_PROXY` entries of the form `*.host.com`
This PR adds an additional check to determine if the proxy should be
bypassed for a given host.
- Closes#255.
- Fixes ProxyUtils so that the environment variable to be used for proxy
selection is determined by the request scheme.
- RegistryClient uses ProxyUtils to get the proxy URL used by the
HTTPClient.
- Tweak hostname resolution error message to avoid misleading output if
the proxy hostname cannot be resolved.
Aditya Ramani