[1.7.x] Fixed #22495 -- Locmem cache.add() failed with infinite timeouts

cache.add() incorrectly succeeded when there was an existing key
with an infinite (None) timeout.
Backport of af5f688392 from master.

tests/cache/tests.py

@@ -491,6 +491,9 @@ class BaseCacheTests(object):
 
         cache.add('key2', 'ham', None)
         self.assertEqual(cache.get('key2'), 'ham')
+        added = cache.add('key1', 'new eggs', None)
+        self.assertEqual(added, False)
+        self.assertEqual(cache.get('key1'), 'eggs')
 
         cache.set_many({'key3': 'sausage', 'key4': 'lobster bisque'}, None)
         self.assertEqual(cache.get('key3'), 'sausage')