Fix RedisCacheStore #write_multi to correctly store entries

[fatkodima]

Fixes #31884.

Before, values were incorrectly stored as strings like
"#<ActiveSupport::Cache::Entry:0x007fd63671af48>". So now they are stored correctly.

I created a test with explicit expires_in as nil to test this code path

rails/activesupport/lib/active_support/cache/redis_cache_store.rb

Lines 361 to 364 in 5d19995

	if mset_capable? && expires_in.nil?
	failsafe :write_multi_entries do
	redis.mapped_mset(entries)
	end

which was always avoided before by tests, because of expires_in here

rails/activesupport/test/cache/stores/redis_cache_store_test.rb

Line 92 in 5d19995

@cache = ActiveSupport::Cache::RedisCacheStore.new(timeout: 0.1, namespace: @namespace, expires_in: 60, driver: DRIVER)

[rails-bot]

r? @schneems

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