Revert "Ticket #2796: don't allow arbitrary overriding of the 'action' class and other parameters pulled from the URL mapper."

This reverts commit 4193a826d3.

index.php

@@ -272,11 +272,7 @@ function main()
         return;
     }
 
-    // Note the order here: arguments from the URL mapper will
-    // override request params that have been sent. This ensures
-    // that for instance an action parameter can't be overridden
-    // with an arbitrary action class.
-    $args = array_merge($_REQUEST, $args);
+    $args = array_merge($args, $_REQUEST);
 
     Event::handle('ArgsInitialize', array(&$args));