Implement support for namespaces in attr binding.

The DOM provides a way to lookup which namespace an attribute name belongs to
given the element the attribute will be bound to, Node.lookupNamespaceURI
(http://www.w3.org/TR/DOM-Level-3-Core/core.html#Node3-lookupNamespaceURI).

Given the namespace, it's simple to add/remove the desired attribute with the
correct namespace using the Node.setElementNS and Node.removeAttributeNS.

src/binding/defaultBindings/attr.js

@@ -5,12 +5,21 @@ ko.bindingHandlers['attr'] = {
         ko.utils.objectForEach(value, function(attrName, attrValue) {
             attrValue = ko.utils.unwrapObservable(attrValue);
 
+            // Find the namespace of this attribute, if any.
+            // Defaulting to `null` should be ok, as
+            //
+            //  element.setAttributeNS( null, name, value ) ~ element.setAttribute( name, value )
+            //  element.removetAttributeNS( null, name ) ~ element.removeAttribute( name )
+            //
+            var prefixLen = attrName.indexOf(':');
+            var namespace = prefixLen < 0 ? null : element.lookupNamespaceURI( attrName.substr(0, prefixLen) );
+
             // To cover cases like "attr: { checked:someProp }", we want to remove the attribute entirely
             // when someProp is a "no value"-like value (strictly null, false, or undefined)
             // (because the absence of the "checked" attr is how to mark an element as not checked, etc.)
             var toRemove = (attrValue === false) || (attrValue === null) || (attrValue === undefined);
             if (toRemove)
-                element.removeAttribute(attrName);
+                element.removeAttributeNS(namespace, attrName);
 
             // In IE <= 7 and IE8 Quirks Mode, you have to use the Javascript property name instead of the
             // HTML attribute name for certain attributes. IE8 Standards Mode supports the correct behavior,
@@ -23,7 +32,7 @@ ko.bindingHandlers['attr'] = {
                 else
                     element[attrName] = attrValue;
             } else if (!toRemove) {
-                element.setAttribute(attrName, attrValue.toString());
+                element.setAttributeNS(namespace, attrName, attrValue.toString());
             }
 
             // Treat "name" specially - although you can think of it as an attribute, it also needs