Remove custom serialization

Some tests are still failing, don't merge yet.

This [test](https://github.com/tmpvar/jsdom/blob/master/test/browser/index.js#L126) isn't correct, I think.
Tried the following in chrome console:
```javascript
var p = document.createElement('p')
p.setAttribute('specials', '"&lt;&gt;&amp;&nbsp;')
p.outerHTML // "<p specials="&quot;&amp;lt;&amp;gt;&amp;amp;&amp;nbsp;"></p>"
```