Fixed incorrect docstring in cache tests (take two)

3e25c8ac69 was an incomplete
merge of the pull request.

Thanks to @ramast for catching this.

tests/cache/tests.py

@@ -496,7 +496,7 @@ class BaseCacheTests(object):
 
     def test_zero_timeout(self):
         '''
-        Passing in zero into timeout results in a value that is cached forever
+        Passing in zero into timeout results in a value that is not cached
         '''
         cache.set('key1', 'eggs', 0)
         self.assertEqual(cache.get('key1'), None)