Spread cache keys when using `Redis#mget`
Spread cache keys when using `Redis#mget`

The [signature of `Redis#mget` actually requires a splatted array of keys to fetch](https://www.rubydoc.info/gems/redis/3.1.0/Redis%3Amget). Currently, the implementation works fine for the default `redis` gem, but the current behavior (non-spreading) does not work with `redis-store`. This patch should fix that.
jnunemaker/flipper,jnunemaker/flipper,jnunemaker/flipper,jnunemaker/flipper