Corrected issue in disabled stash test.

One of the tests checking whether stash was properly being disabled was
not actually having it's assertions tested. This wasn't critical due to
a more comprehensive test that is run, but should be corrected.

tests/Stash/Test/AbstractCacheTest.php

@@ -336,6 +336,7 @@ public function testDisable()
     {
         $stash = $this->testConstruct(array('path', 'to', 'key'));
         $stash->disable();
+        $this->assertDisabledStash($stash);
     }
 
     public function testDisableCacheWillNeverCallDriver()