Add ActiveSupport::Cache::MemCacheStore#inspect

Make the output more readable.
This is a follow up of rails#47944

Author: guilleiguaran

activesupport/lib/active_support/cache/mem_cache_store.rb

@@ -131,14 +131,19 @@ def initialize(*addresses)
           MSG
           @data = addresses.first
         else
-          mem_cache_options = options.dup
+          @mem_cache_options = options.dup
           # The value "compress: false" prevents duplicate compression within Dalli.
-          mem_cache_options[:compress] = false
-          (OVERRIDDEN_OPTIONS - %i(compress)).each { |name| mem_cache_options.delete(name) }
-          @data = self.class.build_mem_cache(*(addresses + [mem_cache_options]))
+          @mem_cache_options[:compress] = false
+          (OVERRIDDEN_OPTIONS - %i(compress)).each { |name| @mem_cache_options.delete(name) }
+          @data = self.class.build_mem_cache(*(addresses + [@mem_cache_options]))
         end
       end
 
+      def inspect
+        instance = @data || @mem_cache_options
+        "#<#{self.class} options=#{options.inspect} mem_cache=#{instance.inspect}>"
+      end
+
       ##
       # :method: write
       # :call-seq: write(name, value, options = nil)