frodsan/json-serializer — pull request #1 by bemurphy (2014-04-20 15:42:01+00:00)
Title: Represent a nil association as json null

If an association was nil, which is not unusual, the existing code would raise
an exception like "NoMethodError: undefined method `id' for nil:NilClass".

Collection associations were already ok because they tend to return something
non-nil like `[]` when they are empty.