Support cucumber tests

[BrianHawley]

Cucumber uses extend, not include.

[BrianHawley]

This change doesn't recommend the use of cucumber for Warden testing; it just enables it.

[BrianHawley]

Here's an example of this in patch form, suitable for putting in an initializer:

if Rails.env.test?
  Warden::Test::Helpers.module_exec do
    if methods(false).exclude?(:extended)
      def self.extended(base)
        included(base)
      end
    end
  end
end

I call included in the patch just in case the code of that method changes.

[BrianHawley]

@jsmestad you merged the last few PRs. Could you take a look at this one?