More informations about `module_function` [here](https://stackoverflow.com/questions/9266308/is-the-use-of-modulemodule-function-considered-bad?answertab=votes#tab-top) and [here](http://ruby-doc.org/core-2.1.1/Module.html#method-i-module_function).

This change is not totally relevant and depends on the way you conceive the use of the `Request` module.
Note that if private methods are introduced in `Doorkeeper::Request` the `module_function` will no longer work for methods relying on those `private` methods.

I could revert the change if you want. I'm not a huge fan of `module_function`, however in that case we should check if it's possible to change the Rubocop's configuration of _HoundCI_.