Backport relation merger updates from 9675c7d to 4-0-stable

Fixes #10995.
Convert Mime::NullType in a singleton

Conflicts:
	actionpack/lib/action_dispatch/http/mime_type.rb
Update guides welcome page to point to version 3.2.16 [ci skip]
Merge pull request #13176 from arunagw/4-0-stable

Update guides welcome page to point to version 3.2.16
Run Travis tests using Ruby 2.1.0-preview2 too
Merge pull request #13183 from sorah/never_ignore_i18n_translate_raise_option

Escalate missing error when :raise is true in translate helper, fix regression introduced by security fix.
Conflicts:
	actionpack/CHANGELOG.md
Cleanup Ruby 2.1 warnings on File.exists?

Mostly cherry-picks from master:

    commit d48222d22ef576c1b429ec2321c712bc221698d7
    Author: laurocaetano
    Date:   Fri Nov 1 18:15:53 2013 -0200

    commit 49062c12edc4b5397ea3bc6680eb6a13bdec2164
    Author: Rashmi Yadav
    Date:   Fri Nov 1 18:07:27 2013 +0100

    commit c10a78124c1c32f8ce3a92888863b9b258efcc06
    Author: Arun Agrawal
    Date:   Fri Nov 1 10:39:06 2013 +0100

    commit 816126862e040b787cf11c8143f326798803f67c
    Author: Arun Agrawal
    Date:   Fri Nov 1 09:49:57 2013 +0100

    commit 4d4ff531b8807ee88a3fc46875c7e76f613956fb
    Author: Aaron Patterson
    Date:   Thu Oct 31 11:47:51 2013 -0700
Fix indent in AP changelog [ci skip]
Merge pull request #12646 from severin/polymorphic_belongs_to_touch

polymorphic belongs_to association with touch: true updates old record correctly
Conflicts:

	activerecord/CHANGELOG.md
	activerecord/lib/active_record/associations/builder/belongs_to.rb
Using 2.3.11 URL for 2.3.x guides [ci skip]
Fix missed stubs of File.exists? when changing to File.exist?

Change to File.exist? to reduce some Ruby 2.1 warnings was introduced
in 4-0-stable branch in 02393966720002314765850776fb16d2539c9415.
Remove deprecated call to Relation#all
Revert "Merge pull request #13149 from laurocaetano/fix_offset_last"

This reverts commit e18b2ca1289d08f76ff3e6ed09f1bf02ab8b7a04.

Reason: this change in behavior may cause some unexpected breakage in
applications relying on a stable version/branch, so we are leaving it
only on Rails master for now (which will be available with the 4.1 release).

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/test/cases/finder_test.rb
Improve AR changelog [ci skip]
Merge pull request #13122 from andrielfn/fix-collection-label-class

Fix the merge of a label class defined inside the block
Conflicts:
	actionpack/test/template/form_collections_helper_test.rb
	actionview/CHANGELOG.md
ActiveRecord::ConnectionAdapters::Column.string_to_time method respects string with timezone.

Closes #12278
Closes #12459

Conflicts:
	activerecord/CHANGELOG.md
Add integration test for #12459
Merge pull request #13207 from georgeguimaraes/backports-12290-to-4-0-stable

Backports #12290 to 4-0-stable. Closes #12459
Merge pull request #12684 from mariovisic/fix_kernel_silence_stream_descriptor_leak

Fix issue with Kernel#silence_stream leaking file descriptors
Conflicts:
	activesupport/CHANGELOG.md
Remove tests backported that should not