Refactor logic to grab the max time of the list of timestamp names in #cache_key

Reuse the already existing logic used for grabbing this information from
the updated columns.

activerecord/lib/active_record/integration.rb

@@ -55,8 +55,8 @@ def cache_key(*timestamp_names)
       when new_record?
         "#{self.class.model_name.cache_key}/new"
       when timestamp_names.any?
-        timestamps = timestamp_names.collect { |method| send(method) }.compact
-        timestamp = timestamps.max.utc.to_s(cache_timestamp_format)
+        timestamp = max_updated_column_timestamp(timestamp_names)
+        timestamp = timestamp.utc.to_s(cache_timestamp_format)
         "#{self.class.model_name.cache_key}/#{id}-#{timestamp}"
       when timestamp = max_updated_column_timestamp
         timestamp = timestamp.utc.to_s(cache_timestamp_format)

activerecord/lib/active_record/timestamp.rb

@@ -98,8 +98,8 @@ def all_timestamp_attributes
       timestamp_attributes_for_create + timestamp_attributes_for_update
     end
 
-    def max_updated_column_timestamp
-      if (timestamps = timestamp_attributes_for_update.map { |attr| self[attr] }.compact).present?
+    def max_updated_column_timestamp(timestamp_names = timestamp_attributes_for_update)
+      if (timestamps = timestamp_names.map { |attr| self[attr] }.compact).present?
         timestamps.map { |ts| ts.to_time }.max
       end
     end