time_expire=0 should clear memcache entry

If time_expire=0, then the cache should be cleared and reset with the
latest value to stay compatible with documented web2py functionality of
how caching works. Previous change broke this compatibility while adding
efficiency in other cases.

gluon/contrib/gae_memcache.py

@@ -32,7 +32,7 @@ class MemcacheClient(object):
 
         key = '%s/%s' % (self.request.application, key)
         value = None
-        obj = self.client.get(key)
+        obj = self.client.get(key) if time_expire != 0 else None
         if obj:
             value = obj[1]
         elif f is not None: