Fix no_proxy behaviour

[christian-van-rensen-tng]

As discussed on this stackoverflow thread, the no_proxy environment variable defines a list of hosts, for which the proxy should be bypassed.
The same semantics should probably apply to the p_no_proxy parameter of Net::HTTP class.

At the moment, this method parameter seems to be misinterpreted as a list of proxies that should not be used.

This behaviour was not detected by the existing testcase, because both the destination hostname and the proxy hostname matched the given exclusion suffix.

It might seem surprising that this problem was not detected before, given the long history of the project.
However, the problem only occurs when the proxy is explicitly configured with `Net::HTTP.new(...)``, but not when using the environment variables.