SKILL-3: As a guest I can receive course details by id

About the type:
I used 'text/plain' because of the arguments in this article:
[http://www.dxw.com/2014/05/rendering-nothing-on-404-in-a-ruby-on-rails-api/]
But I am not sure if those arguments are valid.

Creation of base class ApiController seems to me as a good idea.