tests: fix req.ip test not working with node v0.12

Currently, test/req.ip.js assumes that the connection between the client
and the server is an IPv4 connection. However, depending on the
configuration of the host where this test runs, the connection can be an
IPv4 one or an IPv6 one using an IPv4 mapped address. In the future, it
could also be a "full" IPv6 connection.

This change makes this test handle any type of address.