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

Add i18n scope to disance_of_time_in_words.

This fixes #733.

Revert "truncate debug.log on each test run"

I'm reverting this since we may want to see SQL results of subsequent tests run, to, per example, copare them.

I didn't realized this first time but @yahonda pointed me out.

This reverts commit 98043c68.

Because if every time `debug.log` is truncated,
developers have no way to see the previous ActiveRecord unit test results.
`debug.log` file can be easily reduced
by executing `$ touch /dev/null > debug.log` periodically.

truncate debug.log on each test run

I think it's going to be too much pain to try to transition the
:active_record load hook from executing against Base to executing
against Model.

For example, after Model is included in Base, and modules included in
Model will no longer get added to the ancestors of Base.

So plugins which wish to be compatible with both Model and Base should
use the :active_record_model load hook which executes *before* Base gets
loaded.

In general, ActiveRecord::Model is an advanced feature at the moment and
probably most people will continue to inherit from ActiveRecord::Base
for the time being.