Avoid infinate loops due to circular association referecnes.

[ileitch]

This commit fixes aSystemStackError which can arise in a situation such as this:

I have not tried with master.

class UserSerializer < AcitveModel::Serializer
  has_one :review
end

class ReviewSerializer < ActiveModel::Serializer
  has_one :user
end

[kurko]

Can someone using 0.9 review this?