Preserve 'extends' when the ShadowDOM polyfill patches document.register

I noticed this bug only when using the shadow-dom polyfill in combination with the native document.register. Here is a sample repro example:

```html
<!DOCTYPE html>
<html><body>
  <script src="ShadowDOM/shadowdom.js"></script>

  <x-b></x-b> <span is="x-c"></span>

  <script>
    var bp = Object.create(HTMLElement.prototype);
    bp.createdCallback = function() { console.log("b created"); };
    var B = document.register('x-b', { prototype: bp });

    var cp = Object.create(HTMLSpanElement.prototype);
    cp.createdCallback = function() { console.log("c created"); };
    var C = document.register('x-c', { prototype: cp, extends: 'span' });
  </script>
```

You'd expect to see: `b created` and `c created`, but only `b created` is called.