AssociationCollection#include? ignoring build associated objects

Hey people,

I've created a patch for this issue:
https://rails.lighthouseapp.com/projects/8994/tickets/3472-associationcollectioninclude-ignoring-build-associated-objects,
and I had some positive reviews about it.

I'm not sure about how to proceed with the process of 'verification'
of the patch, but I understand I have to inform here so you are aware
of it (as mentioned on contributor guide).

Let me know what you think about it.

Thanks,
  Marcelo Giorgi