Preserve port and user/pass info for path-based resources

User credentials and port were not being preserved when pre-fetching path-based resources.  For example, consider the following HTML:

```html
<html>
  <body>
    <img src="logo.png" />
  </body>
```

Running the following command would try to fetch the image from `http://localhost/my-site/logo.png` and crash the browser with ConnectionRefused error:
```
$ ./servo http://foo:bar@localhost:8080/my-site/
```

This should now be fixed.