define attribute methods in `init_from_db`

Now that `allocate` is removed, we need to define attribute methods in
all "init" methods.

activerecord/lib/active_record/core.rb

@@ -356,6 +356,8 @@ def init_from_db(attributes)
       @new_record = false
       @attributes = attributes
 
+      self.class.define_attribute_methods
+
       yield self if block_given?
 
       _run_find_callbacks