Fixes a bug with using a variable which doesn't exist.

I kept getting this error:

2013-03-05T10:54:18+00:00 ERR (3): Notice: Undefined variable: block  in /var/www/shop/extensions/magento-turpentine-0.5.0/app/code/community/Nexcessnet/Turpentine/Model/Observer/Esi.php on line 152

So i've changed the usage of $block to $blockObject like in the other log calls.