Replace ControllerConfiguration.new by a factory method for testability.

[ReneB]

Currently, the tests say

it "yields the new instance to the builder block" do
  expect { |block|
      Arrthorizer::Rails::ControllerConfiguration.new(&block)
  }.to yield_with_args(an_instance_of(Arrthorizer::Rails::ControllerConfiguration))
end

This is not entirely accurate. If we replace the constructor with a factory method, we can stub the result from new and have more control over what we're testing.