don't try to remove namespace from a redis future

The commands `blpop`, `keys`, and `mapped_mget` all need the namespace to be removed after the command, but during a `multi` call, we get back `Redis::Future` objects, which will currently cause an error when we try to remove the namespace from it.

There are other ways to handle it, we could implement our own `Future` object as a wrapper around the `Redis` one that removes the namespace when `#value` gets called, but that'd be slightly more refactoring. I'm interested in doing it if anyone is interested in having it, but I thought I'd propose the simplest change first.