Merge pull request #61 from victor-homyakov/patch-17

Optimize the case when `Element.down` is called without any arguments.

src/prototype/dom/dom.js

@@ -1775,6 +1775,7 @@
    *      // -> undefined
   **/
   function down(element, expression, index) {
+    if (arguments.length === 1) return firstDescendant(element);
     element = $(element), expression = expression || 0, index = index || 0;
 
     if (Object.isNumber(expression))