Use document fragments to write multiple nodes

For writer functions that need to generate multiple nodes, the
createDocumentFragment method can be used.

lib/OpenLayers/Format/XML.js

@@ -230,6 +230,26 @@ OpenLayers.Format.XML = OpenLayers.Class(OpenLayers.Format, {
         return element;
     },
 
+    /**
+     * APIMethod: createDocumentFragment
+     * Create a document fragment node that can be appended to another node
+     *     created by createElementNS.  This will call 
+     *     document.createDocumentFragment outside of IE.  In IE, the ActiveX
+     *     object's createDocumentFragment method is used.
+     *
+     * Returns:
+     * {Element} A document fragment.
+     */
+    createDocumentFragment: function() {
+        var element;
+        if (this.xmldom) {
+            element = this.xmldom.createDocumentFragment();
+        } else {
+            element = document.createDocumentFragment();
+        }
+        return element;
+    },
+
     /**
      * APIMethod: createTextNode
      * Create a text node.  This node can be appended to another node with