fix port parsing in extra::url::from_str

[mihneadb]

Before this PR,

extern mod extra;
use extra::url;

fn main() {
    let str = ~"http://localhost:8000";
    let url = url::from_str(str);
    printfln!("%?", url);
}

yields

Ok({scheme: ~"http", user: None, host: ~"localhost", port: Some(~"800"), path: ~"", query: ~[], fragment: None})

After, we get:

Ok({scheme: ~"http", user: None, host: ~"localhost", port: Some(~"8000"), path: ~"", query: ~[], fragment: None})

[mihneadb]

Dupe of #8616 . Closing.