User class checks: Use duck typing

If `Alchemy.user_class` is `nil`, which it is in case of an invalid
configuration, it's not a class, and we can't use the `<` operator.
Let's use duck typing instead.

Author: mamhoff

app/controllers/alchemy/admin/base_controller.rb

@@ -80,13 +80,13 @@ def show_error_notice(error)
       end
 
       def set_stamper
-        if Alchemy.user_class < ActiveRecord::Base
+        if Alchemy.user_class.respond_to?(:stamper=)
           Alchemy.user_class.stamper = current_alchemy_user
         end
       end
 
       def reset_stamper
-        if Alchemy.user_class < ActiveRecord::Base
+        if Alchemy.user_class.respond_to?(:reset_stamper)
           Alchemy.user_class.reset_stamper
         end
       end

app/models/alchemy/folded_page.rb

@@ -16,7 +16,7 @@ class FoldedPage < BaseRecord
     belongs_to :user, inverse_of: :folded_pages, class_name: Alchemy.user_class_name
 
     def self.folded_for_user(user)
-      return none unless Alchemy.user_class < ActiveRecord::Base
+      return none unless Alchemy.user_class.respond_to?(:where)
 
       where(user: user, folded: true)
     end

spec/models/alchemy/page_spec.rb

@@ -1213,25 +1213,19 @@ module Alchemy
           end
         end
 
-        context "with user is a active record model" do
-          before do
-            allow(Alchemy.user_class).to receive(:<).and_return(true)
+        context "if page is folded" do
+          let!(:folded_page) do
+            Alchemy::FoldedPage.create!(page: page, user: user, folded: true)
           end
 
-          context "if page is folded" do
-            let!(:folded_page) do
-              Alchemy::FoldedPage.create!(page: page, user: user, folded: true)
-            end
-
-            it "should return true" do
-              is_expected.to eq(true)
-            end
+          it "should return true" do
+            is_expected.to eq(true)
           end
+        end
 
-          context "if page is not folded" do
-            it "should return false" do
-              is_expected.to eq(false)
-            end
+        context "if page is not folded" do
+          it "should return false" do
+            is_expected.to eq(false)
           end
         end
       end