RedisCache fails to delete cached objects when f=None

[abastardi]

For background, see https://groups.google.com/d/msg/web2py/1LoXXrPlIGE/bX2u3CJ2BQAJ.

According to the book, calling cache.model(key, f=None) should cause key to be deleted from the cache. However, in gluon.contrib.redis_cache.RedisClient.__call__, setting f=None only deletes the cached object if it has expired.

This causes a problem with @cache.action because it relies on the f=None deletion method to delete pages that were cached with an invalid HTTP status.

We should change the behavior of the Redis cache to match that of the other cache models (i.e., setting f=None should always delete the key from the cache).

Also, for extra safety, perhaps @cache.action should delete pages with an invalid status via cache_model.clear(r'^%s$' % cache_key) rather than cache_model(cache_key, None).