Fix AS::NumberHelper results with rationals

:precision was incorrectly being applied to Rationals

before:
  ActiveSupport::NumberHelper.number_to_rounded Rational(10, 3), precision: 2
  => "3.3"
after:
  ActiveSupport::NumberHelper.number_to_rounded Rational(10, 3), precision: 2
  => "3.33"
Create Changing Columns on migrations guides

[ci skip]
Add Total of grouped items to AR query interface on guides

[ci skip]
Update documentation to reflect unsubscription with name.
Merge pull request #15459 from maurogeorge/add-total-grouped-items

Add Total of grouped items to Active Record query interface on guides [ci skip]
Remove docs regarding grouping multiple values, as we should avoid
duplicating API reference in guides. Also :scissors: [ci skip]
Merge pull request #15457 from maurogeorge/add-changing-columns

Create Changing Columns on migrations guides [ci skip]
:nail_care: for migrations guide from #15457 [ci skip]
Avoid looping through subscribers when unsubscribing with a subscriber object.
New records should remain new after yaml serialization
Merge pull request #15444 from sgrif/sg-yaml-new-record

New records should remain new after yaml serialization
Feature requests should be made on the mailing list, not submitted to
the issue tracker. See also #15455 [ci skip]
pg, inline casting methods into `OID::Type` objects.

This inlines casting for the most obvious types. The rest will
follow eventually. I need to put some tests in place, to make sure
that the inlining is not causing regressions.

/cc @sgrif
test pg, add basic test cases for point datatype.
docs, fix typo [ci skip]
Merge pull request #15464 from tgxworld/improve_notifications_logic

Improvements to ActiveSupport::Notifications.
Merge pull request #15379 from xuanxu/rational_precision

Fix AS::NumberHelper results with rationals
just use `assert`
File.exist? instead of File.exists? in bin/setup

File.exists? is deprecated in Ruby 2.1+

https://github.com/ruby/ruby/blob/v2_1_2/file.c#L1413
Add missing colon to internal