CacheEntry.isExpired: fix expiration condition

Maybe this is enough to fix testCacheTool.test_06_CheckCacheExpiration

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@43421 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/CachePlugins/BaseCache.py

@@ -59,7 +59,7 @@ class CacheEntry(object):
       - None means allways expire
       - 0 means never expire
     """
-    return self.expires_at is None or self.expires_at != 0 and self.expires_at < time()
+    return self.expires_at is None or 0 < self.expires_at <= time()
 
   def getValue(self):
     """ return cached value """