Update key existence check to use newer #exists? if available

[zachmargolis]

• Silences deprecation warning starting in redis gem v4.2

In upgrading the redis gem in our project, we get this deprecation warning:

`Redis#exists(key)` will return an Integer in redis-rb 4.3. `exists?` returns a boolean, you should use it instead. To opt-in to the new behavior now you can set Redis.exists_returns_integer =  true. To disable this message and keep the current (boolean) behaviour of 'exists' you can set `Redis.exists_returns_integer = false`, but this option will be removed in 5.0. (/.../ruby/gems/2.6.0/gems/redis-session-store-0.11.1/lib/redis-session-store.rb:70:in `session_exists?')

[renchap]

This is preventing us to upgrade our redis gem. Is anything needed to get this merged?

[Jesterovskiy]

Nothing) Thank you @zachmargolis for code and @renchap for reminding me to merge)

[zachmargolis]

@Jesterovskiy thanks for merging!

I noticed in the v11.2 bump commit that actionpack was pinned to ~> 3, was that intentional? We use Rails 5 in our app so we can't use that version.

It looks like v11.1 specifically unpinned that actionpack dependency?

[Jesterovskiy]

@zachmargolis thanks, fixed)