Introduce `Rails.gem_version`

This method return `Gem::Version.new(Rails.version)`, suggesting a more
reliable way to perform version comparison.

Example:

    Rails.version #=> "4.1.2"
    Rails.gem_version #=> #<Gem::Version "4.1.2">

    Rails.version > "4.1.10" #=> false
    Rails.gem_version > Gem::Version.new("4.1.10") #=> true
    Gem::Requirement.new("~> 4.1.2") =~ Rails.gem_version #=> true

This was originally introduced as `.version` by @charliesome in #8501
but got reverted in #10002 since it was not backward compatible.

Also, updating template for `rake update_versions`.
Merge pull request #14101 from sikachu/master-fix-versioning-task

Introduce `Rails.gem_version`
Update doc for TagHelper.
[ci skip] Add "Low-Level Caching" part to "Caching With Rails"
Fix guide about `config.active_record.default_timezone` [ci skip]

Now the initial value of ActiveRecord.default_timezone is always :utc
Merge pull request #14296 from joker1007/fix_rails_guides_about_active_record_default_timezone

Fix guide about `config.active_record.default_timezone` [ci skip]
Merge pull request #14285 from sikachu/master-revert-render-body-remove-content-type

Do note remove `Content-Type` when `render :body`
Merge pull request #14257 from killthekitten/patch-1

Add "Low-Level Caching" part to "Caching With Rails" guide
unscope doesn't remove only the default_scope, but all of them.

[ci-skip]
Closes rails/rails#14294
fix typo app -> all

Thank you @bquorning
Merge branch 'master' of github.com:rails/docrails

Conflicts:
	guides/source/4_1_release_notes.md
Fix OrderedHash.select to return self instance.

On ruby 2.1.1 the behavior of .select and .reject has changed.
They will return a Hash new instance, so we need to override them to
keep the instance object class.
Add preventDefault() on click event
Merge pull request #14316 from the4dpatrick/master

Add preventDefault() on click event
Make sure we clean preview_interceptors

We should make sure we clean preview_interceptors after each test so they dont leaky to the next test
Merge pull request #14318 from arthurnn/clean_preview_interceptors

Make sure we clean preview_interceptors
Tests for indifferent access using full_messages_for. Closes #11916
added CSRF helper to rails guide documentation. Closes #11615. [ci skip]
docs, shorten CsrfHelper description and refer to security guide. [ci skip]
Make the rails:template rake task load initializers

Templates could rely on irregular inflections or external libraries for
instance so we should load the application's initializers when running
the rails:template task.

The introducing commit of this feature is f7f11361 ; the initializers
have never been loaded invoking this task.

Fixes #12133.