Set explicit string key for Rails cache (#123)

Thanks to rails/rails#50008,
I found that the Rails cache key should be treated as String.
This patch changes the Rails cache to set String instead of Integer
to follow the expected cache key type.

test/dummy/app/controllers/users_controller.rb

@@ -70,7 +70,7 @@ def welcome_email
 
   def cache
     user = User.find(params.require(:user_id))
-    result = Rails.cache.fetch(user.id, expires_in: 1.minutes) do
+    result = Rails.cache.fetch("key_#{user.id}", expires_in: 1.minutes) do
       user.slow_method
     end
     logger.info(result)

test/rails_band/active_support/event/cache_generate_test.rb

@@ -82,7 +82,7 @@ class CacheGenerateTest < ActionDispatch::IntegrationTest
   test 'returns key' do
     get "/users/#{@user.id}/cache"
 
-    assert_equal @user.id, @event.key
+    assert_equal "key_#{@user.id}", @event.key
   end
 
   if Gem::Version.new(Rails.version) >= Gem::Version.new('6.1')

test/rails_band/active_support/event/cache_read_test.rb

@@ -82,7 +82,7 @@ class CacheReadTest < ActionDispatch::IntegrationTest
   test 'returns key' do
     get "/users/#{@user.id}/cache"
 
-    assert_equal @user.id, @event.key
+    assert_equal "key_#{@user.id}", @event.key
   end
 
   if Gem::Version.new(Rails.version) >= Gem::Version.new('6.1')