Middleware is currently registered in config/application.rb using a string referring to the class name, but this is deprecated in Rails 5.0:

2016-09-15T14:29:33  [app] [W] DEPRECATION WARNING: Passing strings or symbols to the middleware builder is deprecated, please change
 | them to actual class references.  For example:
 | 
 |   "Middleware::CatchJsonParseErrors" => Middleware::CatchJsonParseErrors
 | 
 |  (called from <top (required)> at /home/dcleal/code/foreman/foreman/config/environment.rb:5)

It will probably be removed in Rails 5.1.