Fixing bug: DOCTYPE was being stripped by parseFromString

This commit resolves a bug which caused the DOCTYPE to be stripped by
DOMParser().parseFromString when parsing HTML documents. The following
test case was used (run in Node.js):

var xmldom = require('xmldom'),
    DOMParser = xmldom.DOMParser,
    XMLSerializer = new xmldom.XMLSerializer(),
    string = '<!doctype
html><html><head></head><body>content</body></html>',
    doc = new DOMParser().parseFromString(string, 'text/html'),
    s = XMLSerializer,
    str = s.serializeToString(doc);
console.log(str);

I'm not sure if my commit is the best way to fix the bug or not, but it seems to work. :)