Better handling of Ctrl-W event

* IE doesn't trigger warning when session is closed
* IE and Firefox actually sends Ctrl-W event to terminal
* In Chrome user still gets warning and event is NOT sent to terminal

shellinabox/root_page.html

@@ -105,9 +105,8 @@
       window.onbeforeunload = function(e) {
         if (typeof window.shellinabox.session != "undefined") {
           return "Are you sure you want to leave this page?";
-        } else {
-          return null;
         }
+        // Don't use explicit return
       }
 
     --></script>