Fix local variable reference

[camertron]

Usually it looks like instances of Proxy are created with an instance of URI, but if you're like me and you tried to instantiate one by hand with a string, you get a NameError in response. The problem is that url is passed into the constructor but URI.parse is passed a variable named uri, which doesn't exist. Not a problem if you instantiate proxies the right way™, but still a bug :)