Restore test deliveries for ActionMailer.
Rename `property` to `attribute`

For consistency with https://github.com/rails/rails/pull/15557
Merge pull request #15558 from sgrif/sg-rename-property

Rename `property` to `attribute`

Conflicts:
	activerecord/lib/active_record/attribute_methods/serialization.rb
	activerecord/lib/active_record/base.rb
fix test cases after #15558 merge to master.
Merge pull request #15566 from kuldeepaggarwal/fix-build

fix test cases after #15558 merge to master.
Add Missing Test Cases for the constantize
Merge pull request #15567 from aditya-kapoor/add-cases

Add Additional Test Cases for the constantize
Handle client disconnect during live streaming

.. even when the producer is blocked for a write.
Merge pull request #15452 from matthewd/live-abort

Handle client disconnect during live streaming
Merge pull request #15545 from zuhao/refactor_actionpack_assert_select_test

Restore test deliveries for ActionMailer.
Ignore distressingly-frequent segfaults on this build

Not sure what's causing them suddenly, but it seems unlikely there's
much we can do about it. For the small amount of coverage this
particular job gains us, it's hurting us more, by making CI failures
unremarkable.
Update migrations.md

default column type modifier
Merge pull request #15571 from kalmanh/patch-1

Update migrations.md
Mail 2.6.1 silences excessive warnings; remove Gemfile hack

Completes https://github.com/rails/rails/pull/15493

Revert "For our build, stick with mail 2.5.x for now"

This reverts commit b8f586a094c104006d29a87fee0d8b48d0af2d14.
Merge pull request #15573 from bf4/mail_2_6_1_released

Mail 2.6.1 silences excessive warnings; remove Gemfile hack