bug: KeyvRedis key prefix namespace behavior is not consistent with Keyv (core)

Duplicate of <https://github.com/jaredwray/keyv/issues/700>. This applies for other adapters besides Redis. The default behavior should match the behavior that comes out of the box in core Keyv regarding namespaces and key prefixing. Otherwise you run into having to create multiple connections because `ioredis` does not support that (see <https://github.com/luin/ioredis/issues/983>). It would be helpful to have notices as to what makes this repo different than @jaredwray's. It says this one is maintained, but there are recent commits to that repo too. In any case, I'd love to have contributor access on GH and npm so I can help fix this stuff. I use np for version releases and abide by semver.