Isolate each specs for cache store (#6450)

The cache store is explicitly used by some specs, but they were not
isolated and therefore not reliable. This fixes the issue by clearing
the cache after each specs.

spec/rails_helper.rb

@@ -51,6 +51,8 @@ RSpec.configure do |config|
   end
 
   config.after :each do
+    Rails.cache.clear
+
     keys = Redis.current.keys
     Redis.current.del(keys) if keys.any?
   end