Allow overriding the serialization rules in MarkupAccumulator

LayoutTests/fast/dom/Element/script-tests/getAttribute-check-case-sensitivity.js

 description(
 "<p>This file test the behaviour of getAttribute with regard to case.</p><p>See Bug 20247: setAttributeNode() does not work when attribute name has a capital letter in it</p>"
 );
 
 function testGetAttributeCaseInsensitive()
 {
     var div = document.createElement('div');
     div.setAttribute("mixedCaseAttrib", "x");
 
     // Do original case lookup, and lowercase lookup.
@@ skipping 117 matching lines @@
     return a.nodeValue;
 }
 
 shouldBe("testAttribNodeNameGetMutate()", '"1"');
 
 var node = document.createElement("div");
 var attrib = document.createAttribute("myAttrib");
 attrib.nodeValue = "XXX";
 node.setAttributeNode(attrib);
 
-shouldBe("(new XMLSerializer).serializeToString(node)", '"<div myAttrib=\\"XXX\\"></div>"');
+shouldBe("(new XMLSerializer).serializeToString(node)", '"<div xmlns=\\"http://www.w3.org/1999/xhtml\\" myAttrib=\\"XXX\\"></div>"');
 shouldBe("node.getAttributeNode('myAttrib').name", '"myAttrib"');
 shouldBe("node.getAttributeNode('myattrib').name", '"myAttrib"');
 shouldBe("attrib.name", '"myAttrib"');