Remove unnecessary URL encoding added by drupal_get_destination()

This isn't a bug as everything is currently working because drupal_goto() includes urldecode(). In D7 drupal_goto() has urldecode() removed and drupal_get_desintation() returns a keyed array so I also don't think this is an issue for D7.

However I think drupal_get_destination() which returns a URL encoded query is being used incorrectly in this D6 version so attached is a patch to refractor the logic. I've updated the comments to explain the changes.