[TEST] Added an unit test for as_indexed_json changed attribute miss because as_json returns symbols instead of strings in rails 3.2

See Rails 3.2 (missing .to_s)
https://github.com/rails/rails/blob/3-2-stable/activemodel/lib/active_model/serialization.rb#L85

vs Rails 4.1
https://github.com/rails/rails/blob/4-1-stable/activemodel/lib/active_model/serialization.rb#L110

Related: #116