use has_attribute? instead of respond_to for after_initialize.

If you write a query with a select clause that does not return destroyed_at, and build models in the call, the .present? call on destroyed_at in the after_initialize will fail with an ActiveModel::MissingAttributeError. This is an example of:

http://stackoverflow.com/questions/6820156/activemodelmissingattributeerror-occurs-after-deploying-and-then-goes-away-aft

Switching to using has_attribute? fixes this.