Relative protocol not honoured in IE

(Found in IE10)

```
new Url('//url-with-relative-protocol.co.uk/blah').toString();
// actual     ->  'url-with-relative-protocol.co.uk/blah'
// expected 1 ->  '//url-with-relative-protocol.co.uk/blah'
// expected 2 ->  [current page's protocol] + '//url-with-relative-protocol.co.uk/blah'
```
