RedisCacheStore: fix `#write_multi` mset serialization

Closes #31886
Fixes #31884

activesupport/lib/active_support/cache/redis_cache_store.rb

@@ -17,6 +17,7 @@
 
 require "digest/sha2"
 require "active_support/core_ext/marshal"
+require "active_support/core_ext/hash/transform_values"
 
 module ActiveSupport
   module Cache
@@ -360,7 +361,13 @@ def write_multi_entries(entries, expires_in: nil, **options)
           if entries.any?
             if mset_capable? && expires_in.nil?
               failsafe :write_multi_entries do
-                redis.mapped_mset(entries)
+                serialized_entries = if options[:raw]
+                  entries.transform_values { |e| e.value.to_s }
+                else
+                  entries.transform_values { |e| serialize_entry(e) }
+                end
+
+                redis.mapped_mset(serialized_entries)
               end
             else
               super

activesupport/test/cache/behaviors/cache_store_behavior.rb

@@ -113,6 +113,16 @@ def test_fetch_multi
     assert_equal("fufu", @cache.read("fu"))
   end
 
+  def test_fetch_multi_without_expires_in
+    @cache.write("foo", "bar")
+    @cache.write("fud", "biz")
+
+    values = @cache.fetch_multi("foo", "fu", "fud", expires_in: nil) { |value| value * 2 }
+
+    assert_equal({ "foo" => "bar", "fu" => "fufu", "fud" => "biz" }, values)
+    assert_equal("fufu", @cache.read("fu"))
+  end
+
   def test_multi_with_objects
     cache_struct = Struct.new(:cache_key, :title)
     foo = cache_struct.new("foo", "FOO!")