The Rails module could be defined by some other gem than rails

kaminari depends on actionpack that in turn depends on rails-html-sanitizer the line https://github.com/amatsuda/kaminari/blob/master/lib/kaminari.rb#L36
returns true because rails-html-sanitizer defines the class Rails::Html::Sanitizer: https://github.com/rails/rails-html-sanitizer/blob/master/lib/rails-html-sanitizer.rb
see: https://github.com/amatsuda/kaminari/issues/796#issue-158648760

closes #796

lib/kaminari.rb

@@ -8,7 +8,7 @@ rescue LoadError
   #do nothing
 end
 
-$stderr.puts <<-EOC if !defined?(Rails) && !defined?(Sinatra) && !defined?(Grape)
+$stderr.puts <<-EOC if !defined?(::Rails::Railtie) && !defined?(Sinatra) && !defined?(Grape)
 warning: no framework detected.
 
 Your Gemfile might not be configured properly.
@@ -33,7 +33,7 @@ require 'kaminari/models/configuration_methods'
 require 'kaminari/hooks'
 
 # if not using Railtie, call `Kaminari::Hooks.init` directly
-if defined? Rails
+if defined? ::Rails::Railtie
   require 'kaminari/railtie'
   require 'kaminari/engine'
 end

spec/generators/views_generator_spec.rb

@@ -1,6 +1,6 @@
 require 'spec_helper'
 
-if defined?(Rails)
+if defined?(::Rails::Railtie)
   require 'rails/generators'
   require 'generators/kaminari/views_generator'
 

spec/helpers/action_view_extension_spec.rb

@@ -1,6 +1,6 @@
 require 'spec_helper'
 
-describe 'Kaminari::ActionViewExtension', :if => defined?(Rails)do
+describe 'Kaminari::ActionViewExtension', :if => defined?(::Rails::Railtie) do
   describe '#paginate' do
     before do
       50.times {|i| User.create! :name => "user#{i}"}

spec/spec_helper.rb

@@ -15,7 +15,7 @@ require 'database_cleaner'
 # Simulate a gem providing a subclass of ActiveRecord::Base before the Railtie is loaded.
 require 'fake_gem' if defined? ActiveRecord
 
-if defined? Rails
+if defined? ::Rails::Railtie
   require 'fake_app/rails_app'
 
   require 'rspec/rails'