add include_hidden option to collection_check_boxes helper
Fix MailerPreview broken tests

`BaseMailerPreview.welcome` is an instance method, so we need to stub the
method on a instance level and not on Class. The stub is important to make
sure the Message object is the same in the other expectations.
This was working randomly because Mocha uses == to compare two objects
on the `with()` expectation and even if the Mail::Message objects were
not the same object they are equal, but thats not the case in 100% of
the runs. So we need to make sure we use `.any_instance` method and have
the right message object.
Merge pull request #12662 from nashby/include-hidden-collection

add include_hidden option to collection_check_boxes helper

Conflicts:
	actionview/CHANGELOG.md
	actionview/test/template/form_collections_helper_test.rb
Merge pull request #14395 from arthurnn/fix_mailer_preview_test

Fix MailerPreview broken tests
Reword 5.6 strong parameters and private method stuff [ci skip]
Merge pull request #14396 from afshinator/master

Reword 5.6 strong parameters and private method stuff [ci skip]
String inflections should use I18n.locale if locale is not specified

When we set languages other than English (:en), we expect every word to
be translated to the language we set. However, String#pluralize and
String#singularize still use :en when locale is not specified. This
means we always need to specify locale every time we use one of these
methods.

To avoid that mess, use `I18n.locale` by default so everything will be
translated properly without specifying locale.