I sent a pull request on the Github repository of rack and it was accepted:
https://github.com/rack/rack/commit/b487f02b13f42c5933aa42193ed4e1c0b90382d7
In a near future, we will be able to use Thin just by adding gem 'thin' to our Gemfile and starting app with rails s.
Note that this may be a temporary measure, however.
I chose Thin because Mongrel was not maintained currently and no other server seemed to suit as an alternative to Mongrel.