Conflicts:
	activerecord/lib/active_record/callbacks.rb

Added missing space in previous hash related commit

I believe the topics and their headlines should be consistent for obvious reasons. Also

Conflicts:
	activesupport/lib/active_support/core_ext/hash/slice.rb
	guides/source/active_support_core_extensions.md

Re-writing the AJAX Guide

check_box value can be not only an object of Array class

there is a chance that `value` is a Set or an object that reponses to `include?` method so let's handle this case

This reverts commit 83846838.

Use `Rails.env` instead of `ENV['RAILS_ENV']`.
Fix behavior of `rake db:structure:load`.

Small change to active_record/locale/en.yml

Moved activerecord.errors.messages.taken to errors.messages.taken so that translations for, e.g., errors.attributes.email.taken don't get overridden.

Test that the translation for 'taken' can be overridden