Don't assign default attributes until after loading schema

If the call to `.define_attribute_methods` actually ends up loading the
schema (*very* hard to do, as it requires the object being created
without `allocate` having been called, but it can be done by manually
calling `initialize` from inside `marshal_load` if you're crazy), the
value of `_default_attributes` will change from that call.

activerecord/lib/active_record/core.rb

@@ -330,8 +330,8 @@ module ActiveRecord
     #   # Instantiates a single new object
     #   User.new(first_name: 'Jamie')
     def initialize(attributes = nil)
-      @attributes = self.class._default_attributes.deep_dup
       self.class.define_attribute_methods
+      @attributes = self.class._default_attributes.deep_dup
 
       init_internals
       initialize_internals_callback