Refactoring and adding tests to Hydra::Ezid.config

This commit actually covers two things:

* Correcting private declaration for module functions
  All _instance_ methods defined after the `private` are marked as
  private; In the case of `private; def self.foo` the foo method is
  a class method.
* Adding a rudimentary spec to determine what is happening in the
  Hydra::Ezid.config method