Use model_class instead of requiring user to pass in class

Signed-off-by: Ben Johnson <bjohnson@binarylogic.com>

shoulda_macros/authlogic.rb

@@ -2,9 +2,10 @@
 
 module Authlogic
   module ShouldaMacros
-    def should_be_authentic(model)
+    def should_be_authentic
+      klass = model_class
       should "acts as authentic" do
-        assert model.respond_to?(:acts_as_authentic_config)
+        assert klass.respond_to?(:acts_as_authentic_config)
       end
     end
   end