Define page method when a AR class is defined

Commit 92052eedf0 looked good at the beginning. However, that causes a bug that kaminari's page_method_name config has no effect since the pagination method will be defined when `lib/kaminari/models/active_record_model_extension.rb` is loaded.

Define that 'page' method when an AR class is actually defined so we will be able to change the name of the method properly. Fixes #481

lib/kaminari/models/active_record_model_extension.rb

@@ -6,17 +6,17 @@ module Kaminari
 
     included do
       self.send(:include, Kaminari::ConfigurationMethods)
-    end
 
-    module ClassMethods
       # Fetch the values at the specified page number
       #   Model.page(5)
-      define_method(Kaminari.config.page_method_name) do |num = nil|
-        limit(default_per_page).offset(default_per_page * ([num.to_i, 1].max - 1)).extending do
-          include Kaminari::ActiveRecordRelationMethods
-          include Kaminari::PageScopeMethods
+      eval <<-RUBY
+        def self.#{Kaminari.config.page_method_name}(num = nil)
+          limit(default_per_page).offset(default_per_page * ([num.to_i, 1].max - 1)).extending do
+            include Kaminari::ActiveRecordRelationMethods
+            include Kaminari::PageScopeMethods
+          end
         end
-      end
+      RUBY
     end
   end
 end

spec/models/active_record/scopes_spec.rb

@@ -2,6 +2,25 @@ require 'spec_helper'
 
 if defined? ActiveRecord
 
+  describe Kaminari::ActiveRecordModelExtension do
+    before do
+      Kaminari.configure do |config|
+        config.page_method_name = :per_page_kaminari
+      end
+      class Comment < ActiveRecord::Base; end
+    end
+
+    subject { Comment }
+    it { should respond_to(:per_page_kaminari) }
+    it { should_not respond_to(:page) }
+
+    after do
+      Kaminari.configure do |config|
+        config.page_method_name = :page
+      end
+    end
+  end
+
   shared_examples_for 'the first page' do
     it { should have(25).users }
     its('first.name') { should == 'user001' }