Implemented Document.contentType

LayoutTests/fast/dom/HTMLDocument/clone-node.html

 <!-- quirks mode -->
 <title>Tests HTMLDocument cloneNode</title>
 <link rel="help" href="http://dom.spec.whatwg.org/#dom-node-clonenode">
 <script src="../../../resources/js-test.js"></script>
 <script>
 description('Tests cloneNode for HTMLDocument.');
 
 function className(object) {
     return Object.prototype.toString.call(object).slice(8, -1);
 }
 
 shouldBe('document.cloneNode(false).__proto__', 'HTMLDocument.prototype');
 shouldBeEqualToString('className(document.cloneNode(false))', 'HTMLDocument');
 shouldBe('document.cloneNode(true).title', 'document.title');
 shouldBeEqualToString('document.cloneNode(true).compatMode', 'BackCompat');
+shouldBeEqualToString('document.cloneNode(true).contentType', 'text/html');
 
 var doc = document.implementation.createHTMLDocument('title');
 shouldBe('doc.cloneNode(false).__proto__', 'HTMLDocument.prototype');
 shouldBeEqualToString('className(doc.cloneNode(false))', 'HTMLDocument');
 shouldBe('doc.cloneNode(true).title', 'doc.title');
 shouldBeEqualToString('doc.cloneNode(true).compatMode', 'CSS1Compat');
+shouldBeEqualToString('doc.cloneNode(true).contentType', 'text/html');
 
 </script>
 </body>