Prevent IE leaks

Without fix:

 - when `ELEMENT_CACHE` is not empty (e.g. `SPAN` was created with `new Element('span'))`, IE will leak memory for `DIV` and `ELEMENT_CACHE` after page unload.
 - when `ELEMENT_CACHE` is empty, `DIV` is cleaned correctly without any additional actions.

Problem detected and fix tested in sIEve-0.0.8.

Author: Victor Homyakov

src/prototype/dom/dom.js

@@ -3468,5 +3468,15 @@
   }
 
   Element.addMethods(methods);
-  
+
+  // Prevent IE leaks on DIV and ELEMENT_CACHE
+  DIV = null;
+
+  function destroyCache_IE() {
+    ELEMENT_CACHE = null;
+  }
+
+  if (window.attachEvent)
+    window.attachEvent('onunload', destroyCache_IE);
+
 })(this);