Add missing require so that DateTime has the right superclass

If the DateTime core extensions were loaded before the Date core extensions
then you would get a superclass mismatch as DateTime hasn't been defined
yet so it gets set to Object by the acts_like core extension.

Fixes rails#11206

Author: pixeltrix

activesupport/lib/active_support/core_ext/date_time/acts_like.rb

@@ -1,3 +1,4 @@
+require 'date'
 require 'active_support/core_ext/object/acts_like'
 
 class DateTime