Fix rails4 model_name compatibility issue

`model_name` method in Rails4 returns ActiveModel::Name instead of String

API Document: http://edgeapi.rubyonrails.org/classes/ActiveModel/Naming.html#method-i-model_name
Actual change committed in rails: https://github.com/rails/rails/commit/72cbccb5f77ebbd56b3b82a3eda99b6d640d8084

To solve this issue, adding `to_s` is the simplest solution I think. So, please merge this.
If you know better solution,  I'm happy to close this issue.