[Bugfix] Fix flushing of a Redis cache section

When I fixed the bug on section flushing in issue #1918, I missed the fact that the redis section still did the old-style flushing. This should fix that, although I have not tested this code because I do not have access to Redis. As I read the code, without this fix, flush will try to increment a string that is not an integer, and will throw an error.