Acceptance test all supported cache store backends

[grzuy]

(follow-up of #293)

As rack-attack developers
We want rack-attack integration with different cache store backends to be tested by acceptance test cases, which test from the gem user perspective
So that we can be fairly confident that there are no user-facing regressions for each merged PR and release
And is clearer which are the actually working cache store backends

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Stores that are being tested in spec/integration/rack_attack_cache_spec.rb

• ActiveSupport::Cache::MemCacheStore (via activesupport) (Acceptance test use of ActiveSupport::Cache::MemCacheStore #343)
• ActiveSupport::Cache::RedisCacheStore (via activesupport) (Acceptance test use of ActiveSupport::Cache::RedisCacheStore #341)
• Dalli::Client (via dalli) (Acceptance test supported cache store backends #360)
• ConnectionPool.new { Dalli::Client.new } (via connection_pool and dalli) (Acceptance test supported cache store backends #360)
• ActiveSupport::Cache::DalliStore (via dalli) (Acceptance test supported cache store backends #360)
• Redis::Store (via redis-store) (Acceptance test supported cache store backends #360)
• ActiveSupport::Cache::RedisStore (via redis-activesupport) (Acceptance test supported cache store backends #360)
• ActiveSupport::Cache::MemoryStore (via activesupport) (Acceptance test supported cache store backends #360)