Fix OmniAuth OAuth2Generic strategy not loading
In https://github.com/rails/rails/commit/83b767ce, Rails 5.1 removed
support for using a
String to specify a middleware. When the
strategy_class argument is passed from the GitLab YAML config to Devise,
Devise passes the string value straight through to Rails, and GitLab
would crash with a
To make this OmniAuth strategy work again, we normalize the arguments by converting the strategy_class value into an actual Class.