During test runs I namespace all Redis operations under "test:* using the redis-namespace Gem. Let's check to see if it works if I configure only: ["test*"]:

$ redis-cli set 'foo' 'bar'
OK
$ redis-cli keys '*'
1) "foo"
$ bin/rspec path/to/spec
# ASSUME TESTS RUN...
$ redis-cli keys '*'
(empty array)

Uh oh! We've accidentally wiped every single key in the Redis DB. This is a bummer for my development environment.

I've tracked this down to this bit of code:

1. On L26 we don't find any keys in the "test:*" namespace as expected. Ergo only == []
2. except: isn't configured, ergo except == []
3. only.none? is true, therefore we go ahead and find all keys in the Redis DB (bug!!)
4. (only - except) returns every single key in the Redis DB, and they're then dutifully wiped out

A reproduction PR: #13

I was hoping we could simply drop L28 but it seems like there's existing behaviour that relies upon this logic.