Instead of generating association methods directly in the model
class, they are generated in an anonymous module which
is then included in the model class. There is one such module
for each association. The only subtlety is that the
generated_attributes_methods module (from ActiveModel) must
be forced to be included before association methods are created
so that attribute methods will not shadow association methods.

This change uses Module.redefine_method as defined in ActiveSupport.
Making Module.define_method public would be as clean in the code, and
would also emit warnings when redefining an association. That is pretty
messy given current tests, so I'm leaving it for someone else to decide
what approach is better.