Ignore AR tests of index comment when using Oracle

Use appropriate type to `header` option

The `header` option checks only whether it is specified or not.
https://github.com/rails/rails/blob/e8c33349bfabca28996ac74d344d69c7aaffec50/railties/lib/rails/commands/dbconsole/dbconsole_command.rb#L52

https://bugs.ruby-lang.org/issues/13239

Don't highlight code in anchorlink [ci skip]

Minor fix to #28662

Downplay links in Guides headers

[ci skip]

Update backtrace cleaner to use `Regexp#match?`

Clear active connections after initialization

Raise NotImplementedError when using empty_insert_statement_value with Oracle

This reverts commit a680a581, reversing
changes made to 842f67dd.

Reason: I can't resist to the joke, so better to keep it there
https://github.com/rails/rails/pull/28598#issuecomment-290945339.

CI against Ruby 2.4.1

Related #28532, #27636

This reverts commit 82a7593e.

Add anchor links to all headers

No need to check nil

```ruby
nil.respond_to?(:stringify_keys) # => false
```

```ruby
include ActiveModel::AttributeAssignment
Object.new.assign_attributes(nil) #  => ArgumentError: When assigning attributes, you must pass a hash as an argument.
```

Limit stale checks to issues

To allow easy linking at all levels and not just from index

This CHANGELOG.md is a continuation of the 5-1-stable one, there
shouldn't be any duplicate entries.

[ci skip]

Add default_scope note to ActiveRecord guide

Adds note to explain unexpected behavior described in issue #28561.
Developers may be surprised that the internal representation of similar
query conditions in Hash and Array formats in default_scope will yield 
different results for model initialization.
Link: https://github.com/rails/rails/issues/28561

Fix a failed AR test when running with OracleAdapter

CI against Ruby 2.3.4

Add missing backtick to deprecation message

https://www.ruby-lang.org/en/news/2017/03/30/ruby-2-3-4-released/

Fix example usage of ActiveRecord::Base.establish_connection [ci skip]

[ci skip]

Restore rails-ujs.js UMD module support

Move HashWithIndifferentAccess tests to separate file