Compressed all images from the guide through ImageOptim which saves sometimes up to 74.6% in size [ci skip]

Remove accidentally pasted NOTE section during introduction of +beginning_of_hour+ and +end_of_hour+ documentation.

Added documentation for +beginning_of_hour+ and +end_of_hour+ to Active Support Core Extensions guide.

The former is grammatically incorrect.

SpawnMethods#merge returns the intersection when passed an array, and not the union. Update the documentation to reflect this.

This reverts commit 7f536f91.

Reason: If at all this is done, this should be done in rails master.

https://github.com/rails/rails/issues/4027

Editing log levels to reflect this doc: http://api.rubyonrails.org/classes/ActiveSupport/BufferedLogger/Severity.html

Documentation fix: Change the example for habtm association to use proc according to changelog

Make ActionController#head pass rack-link

Provided fix for calling rake tasks within mountable engines

Fix cache (FileStore) clear to keep .gitkeep.

Cover Rails::DBConsole with tests

Active Support Cache - race_condition_ttl should be an integer

RouteSet: optimize routes generation when globbing is used

Allows assert_redirected_to to accept a regular expression

use extract_options!

Minor ActionMailer tweaks