TypeError when Document is created with DocumentType

Example of error:
```js
var xmldom = require('xmldom');
var impl = new xmldom.DOMImplementation();

var doctype = impl.createDocumentType('svg', '-//W3C//DTD SVG 1.1//EN', 'http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd');
var doc = impl.createDocument('http://www.w3.org/2000/svg', 'svg', doctype);

console.log((new xmldom.XMLSerializer).serializeToString(doc));
```

Throws:
```
/Users/ing/Sites/xmldom/dom.js:462
                cs[i++] = child;

TypeError: Cannot set property '0' of null
    at _onUpdateChild (/Users/ing/Sites/xmldom/dom.js:462:13)
    at _insertBefore (/Users/ing/Sites/xmldom/dom.js:530:2)
    at Document.insertBefore (/Users/ing/Sites/xmldom/dom.js:580:10)
    at Document.Node.appendChild (/Users/ing/Sites/xmldom/dom.js:334:15)
    at Object.DOMImplementation.createDocument (/Users/ing/Sites/xmldom/dom.js:272:8)
    at Object.<anonymous> (/Users/ing/Sites/sprites/src/lib/test.js:5:16)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
```
Merge pull request #78 from isnotgood/master

TypeError when Document is created with DocumentType
thanks for your work