Pass default port if port is nil

Addressable::URI returns nil for  port, if it is not provided in an URI
So, ```.default_port``` should be used instead

Because of this, URIs without port part do not work with SOCKS proxies.

    > Addressable::URI.parse('http://test.ru').port
    => nil
    > Addressable::URI.parse('http://test.ru').default_port
    => 80

Ref: https://github.com/sporkmonger/addressable/issues/179
Can this be simplified to `client.req.uri.inferred_port`?