Fix issue with Element.visible reading only inline style. [closes prototypejs#131]

Author: savetheclocktower

src/prototype/dom/dom.js

@@ -290,7 +290,7 @@
    *      // -> true
   **/
   function visible(element) {
-    return $(element).style.display !== 'none';
+    return $(element).getStyle('display') !== 'none';
   }
 
   /**

test/unit/dom_test.js

@@ -290,6 +290,10 @@ new Test.Unit.Runner({
   testElementVisible: function(){
     this.assertNotEqual('none', $('test-visible').style.display);
     this.assertEqual('none', $('test-hidden').style.display);
+    
+    this.assert($('test-visible').visible());
+    this.assert(!$('test-invisible').visible());
+    this.assert(!$('test-hidden').visible());
   },
 
   testElementToggle: function(){

test/unit/fixtures/dom.html

@@ -2,7 +2,12 @@
   <p id="scroll_test_2">Scroll test</p>
 </div>
 
+<style type="text/css">
+  #test-invisible { display: none; }
+</style>
+
 <div id="test-visible">visible</div>
+<div id="test-invisible">invisible</div>
 <div id="test-hidden" style="display:none;">hidden</div>
 <div id="test-toggle-visible">visible</div>
 <div id="test-toggle-hidden" style="display:none;">hidden</div>