* master: (24 commits)
  unscope should remove bind values associated with the where
  reverse_order_value= is not private, so no need to send
  avoid more dynamic symbols
  no need to to_sym
  recover from test runs that leave the database in a bad state
  updates screenshot data [ci skip]
  "serie" => "series"
  revises a few things in the getting started guide [ci skip]
  Favor canonical environment variables for secrets
  removed extra comma [ci skip]
  Only lookup `config.log_level` for stdlib `::Logger`. Closes #11665.
  Updated Changelog to reflect removal of :dependent => :restrict
  standardize on jruby_skip & rbx_skip
  fix bug in becomes! when changing from base to subclass. Closes #13272.
  highlight http://localhost:3000 in README.md. Closes #13643. [ci skip]
  doc proc/lambda arg on inclusion validation. Closes #13689. [ci skip]
  Skip Spring App Generator tests on JRuby
  fixes a typo in a CHANGELOG
  upgrade SDoc
  fixes the Gemfile generator templates
  ...

Conflicts:
	activerecord/test/cases/hot_compatibility_test.rb

It's not public API, but is not private scoped.

"serie" => "series"

More to come.

Favor canonical environment variables for secrets

Prefixing an environment variable with `RAILS_` should be used when there is otherwise a conflict, such as `RAILS_ENV` or if it is being used for a very Rails specific value.

If we remove the prefix of `RAILS_` in the case of `RAILS_SECRET_KEY_BASE` then we can push for a pseudo standard among other frameworks that will accept a common environment key `SECRET_KEY_BASE` to keep your app secure. This is beneficial for containerized deployments such as docker, Heroku, etc. So that the container need to know one less thing about your app (it can just set it no-matter what language or framework you are using). 

This change also allows the defaults to be consistent with the way the secret key is accessed so `secrets.secret_key_base` is aliased to `SECRET_KEY_BASE` instead of `RAILS_SECRET_KEY_BASE`.

removed extra comma [ci skip]

This prevents Rails from assigning meaningless log levels to third
party loggers like log4r. If `Rails.logger` is not `kind_of?(::Logger)`
we simply assign the `config.log_level` as is.

This bug was introduced by #11665.

Updated Changelog to reflect removal of :dependent => :restrict

Based on https://github.com/rails/rails/issues/2502, the option has
been removed.

[JRuby] Fix incorrect RUBY_ENGINE value 

This Adds helpers(jruby_skip & rbx_skip). In Future, Plan is to use
these helpers instead of calls directly to
RUBY_ENGINE/RbConfig/JRUBY_VERSION

This has no effect on the API docs. The main API page is taken from:
    railties/RDOC_MAIN.rdoc

Skip Spring App Generator tests on JRuby

Clarify behavior of json_escape [ci skip]

use enum labels as form values. Achieved by `_before_type_cast`.

Spring makes extensive use of Process.fork, so won't be able to provide
JRuby Support, hence skip these tests on JRuby.

Kudos to @zzak for taking over SDoc and make it work with RDoc 4.

The templates were written as if for a given gem you could either pass a version
or options, but not both. But you may want to specify a version and also a group
or whether the gem has to be required, for example.

Fix iterating over DateTime by doing strict checking for Time objects

* master: (2794 commits)
  doc, API example on how to use `Model#exists?` with multiple IDs. [ci skip]
  Restore DATABASE_URL even if it's nil in connection_handler test
  [ci skip] - error_messages_for has been deprecated since 2.3.8 - lets reduce any confusion for users
  Ensure Active Record connection consistency
  Revert "ask the fixture set for the sql statements"
  Check `respond_to` before delegation due to: https://github.com/ruby/ruby/commit/d781caaf313b8649948c107bba277e5ad7307314
  Adding Hash#compact and Hash#compact! methods
  MySQL version 4.1 was EOL on December 31, 2009 We should at least recommend modern versions of MySQL to users.
  clear cache on body close so that cache remains during rendering
  add a more restricted codepath for templates fixes #13390
  refactor generator tests to use block form of Tempfile
  Fix typo [ci skip]
  Move finish_template as the last public method in the generator
  Minor typos fix [ci skip]
  make `change_column_null` reversible. Closes #13576.
  create/drop test and development databases only if RAILS_ENV is nil
  Revert "Speedup String#to"
  typo fix in test name. [ci skip].
  `core_ext/string/access.rb` test what we are documenting.
  Fix typo in image_tag documentation
  ...

Conflicts:
	activerecord/lib/active_record/associations/join_dependency/join_association.rb
	activerecord/lib/active_record/relation/query_methods.rb