Fix redis-activesupport with ActiveSupport 7.1.1

Otherwise you'll see test failures like, eg,

```
NoMethodError: undefined method `present?' for 2:Integer

          if options[:expires_in].present? && options[:race_condition_ttl].present? && options[:raw].blank?
                                 ^^^^^^^^^
    /Users/jon/.gem/ruby/p0vc9077im6p4r2hwmjldm9qjdksn06v-ruby-3.1.4/gems/redis-activesupport-5.3.0/lib/active_support/cache/redis_store.rb:82:in
    `block in write'
```

spec/acceptance/stores/active_support_redis_store_spec.rb

@@ -5,6 +5,9 @@
 if defined?(::ActiveSupport::Cache::RedisStore)
   require_relative "../../support/cache_store_helper"
   require "timecop"
+  # redis-activesupport has some undeclared dependencies on ActiveSupport:
+  require "active_support/core_ext/object/blank"
+  require "active_support/core_ext/hash/keys"
 
   describe "ActiveSupport::Cache::RedisStore as a cache backend" do
     before do