Use IN operator like Arel for empty hash in where clause

Fix #9168 Initialize NullCookieJar with all options needed for KeyGenerator

Describe better the changes related to raising errors with empty
array/hash values in where clauses.

[ci skip]

Activer Record:  Change behaviour with empty array in where clause

Active Record: Change behaviour with empty hash in where clause

Fix article for generator name

active_record: Quote numeric values compared to string columns.

Travis failed because the postgresql version installed there does not
support extensions, so we just need to skip this for now.

There is no need to create the extension in the database just to test if
it's dumped, we can stub that instead.

When extensions are supported but there's no one enabled in the
database, we should not print anything related to them in schema.rb.

[ci skip]

Adding database extension support to schema.rb

add ActiveRecord::AbstractAdapter#extensions and ActiveRecord::ConnectionAdapters::PostgreSQLAdapter#extensions to allow dumping of enabled extensions to schema.rb, add ActiveRecord::SchemaDumper#extensions to dump extensions to schema.rb

improve tests to check for existence of extensions method, and skip testing dumped extensions if they are unsupported by the database

And improve AS changelog a bit [ci skip]

Modify TimeWithZone#as_json to return 3DP of sub-second accuracy.

[ci skip]

ruby constant syntax is not supported as routing `:controller` option.

The current implementation only works correctly if you supply the `:controller`
with directory notation (eg. `:controller => 'admin/posts'`).

The ruby constant notation (eg. `:controller => 'Admin::Posts`) leads to unexpected problems with `url_for`.

This patch prints a warning for every non supported `:controller` option. I also added documentation how
to work with namespaced controllers. The warning links to that documentation in the rails guide.

Use 'some' instead of 'a/an'.

Get rid of old .rvmrc and .rbenv-version files.

Ignore .ruby-version

Update upgrading guide with ActionDispatch::BestStandardsSupport removal [ci skip]