fix bug mootools#2310

IE7 clone node
(http://msdn.microsoft.com/en-us/library/ie/ms536365(v=vs.85).aspx)
have problem when cloning element with id attribute stetted.

Source/Element/Element.js

@@ -818,7 +818,16 @@ Element.implement({
 
 	clone: function(contents, keepid){
 		contents = contents !== false;
+		var oldID;
+		/*<ltIE8>*/
+		if(Browser.ie6 || Browser.ie7 && !keepid){
+			oldID = this.removeAttribute('id');
+		}
+		/*</ltIE8>*/
 		var clone = this.cloneNode(contents), ce = [clone], te = [this], i;
+		/*<ltIE8>*/
+		oldID && Browser.ie6 || Browser.ie7 && this.setAttribute('id', oldID);
+		/*</ltIE8>*/
 
 		if (contents){
 			ce.append(Array.from(clone.getElementsByTagName('*')));