Do not try to encoding the parameters when the controller is not defined

Set consistent typecast ENV["VERBOSE"]

Pass request params to ActionMailer::Preview

Remove useless test case

Fix `current_page?` regression:

Remove mysql2 database adapter default username root

When you have a route that points to an nonexistent controller we raise
an exception.

This exception was being caught by the DebugExceptions middleware in
development, but when trying to render the error page, we are reading
the request format[[1][]]. To determine the request format we are reading
the format parameters[[2][]], and to be able to read the parameters we need
to encode them[[3][]]. This was raising another exception that to encode the
parameter we try to load the controller to determine if we need to
encode the parameters are binary[[4][]]. This new exception inside the
DebugExceptions middleware makes Rails to render a generic error page.

To avoid this new exception now we only encode the parameters when the
controller can be loaded.

Fixes #28892

[1]: https://github.com/rails/rails/blob/f52cdaac6336f99d13622ff9bda556a3124a4121/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb#L80
[2]: https://github.com/rails/rails/blob/f52cdaac6336f99d13622ff9bda556a3124a4121/actionpack/lib/action_dispatch/http/mime_negotiation.rb#L63
[3]: https://github.com/rails/rails/blob/f52cdaac6336f99d13622ff9bda556a3124a4121/actionpack/lib/action_dispatch/http/parameters.rb#L58
[4]: https://github.com/rails/rails/blob/f52cdaac6336f99d13622ff9bda556a3124a4121/actionpack/lib/action_dispatch/http/parameters.rb#L88

[ci skip]

@dhh most changes that I mentioned in the talk were already there:
https://speakerdeck.com/claudiob/rails-5-dot-1-upcoming-features

I just added a few that were missing.

Some improvements to the 5.1 release notes

Revert "Merge pull request #27636 from mtsmfm/disable-referential-integrity-without-superuser-privilege-take-2"

This reverts commit c1faca63, reversing
changes made to 8c658a0e.

See https://github.com/rails/rails/pull/27636#issuecomment-297534129

- `check_parameters` kwargs was added to the `current_page?` method, the implementation was assuming only hashes responds to `delete`. This was causing issues when `current_page?` was called with a Active Model object
- ref https://github.com/rails/rails/pull/27549
- Fixes #28846

Add cases to ensure that environment variables VERBOSE and VERSION have
correct typecast.

Set consistent type cast ENV["VERBOSE"]:
  ENV["VERBOSE"] is true if it not equal "false"

[ci skip]

Cannot call private methods in `@klass` against `CollectionProxy`
(inherites `Relation`) because using `public_send` in `method_missing`.

Ensure bin/yarn matches the one generated by webpacker

Mixin `CollectionProxy::DelegateExtending` after `ClassSpecificRelation`

`respond_to_missing?` should be private

Fix MigratorTest#test_migrator_verbosity

Add `ActiveRecord::Migration.verbose = true`
to be sure that verbose is turned on in the test.

Related to #28865

Ask for a sane version of SDoc

Replace \Z to \z

Remove checks for Enumerator#size method

Fix typos [ci skip]

'lookup' is the noun. 'to look up' is the verb. Looked it up just to
be sure.

cf.
https://en.wiktionary.org/wiki/lookup
https://en.wiktionary.org/wiki/look_up

The Enumerator#size method was introduced in Ruby 2.0.

These tests were added when Rails 4.1 was current, and Ruby 1.9.3 was
still supported. Since Rails 5 only Ruby >= 2.2.2 is supported, so the
checks are no longer necessary.

[ci skip]

Unify the name of GitHub [ci skip]

Add `form_with_generates_remote_forms` to configuring guide [ci skip]

\Z was a mistake of \z. Replace \Z to \z to prevent newly \Z added.

Follow up of #17148.

[ci skip]