Merge pull request #15521 from edogawaconan/fix_bc_postgres_master

Fix BC year handling in postgres
Merge pull request #15529 from zuhao/refactor_actionpack_mime_type_test

Make sure Mime::Type is not altered after tests.
Reset callbacks after test.

Otherwise the state of callback chain is leaked.
Avoid hard-coded value in test setup/teardown.
Restore Response.default_headers after test.
Merge pull request #15531 from zuhao/refactor_actionpack_request_test

Avoid hard-coded value in test setup/teardown.
Merge pull request #15532 from zuhao/refactor_actionpack_response_test

Restore Response.default_headers after test.
Merge pull request #15523 from gregmolnar/doc_fix

document include_blank's usage with a string [ci skip]
Merge pull request #15513 from zenspider/remove_flush_output_buffer

Removed CaptureHelper#flush_output_buffer as it is only used in tests.
Merge pull request #15530 from zuhao/refactor_actionpack_reloader_test

Reset callbacks after test.
Avoid misuse of underscore argument

Per convention, underscore-only argument names should be used for unused parameters.
Merge pull request #15535 from coreyward/patch-1

Avoid misuse of underscore argument
Remove optimization that was required with whiny nils

Whiny nils is no longer a thing, so we no longer need this optimization
Merge pull request #15536 from sgrif/sg-remove-whiny-nil-thing

Remove optimization that was required with whiny nils
Prevent state leak.
Remove redundant code.
Use a frozen Set instance for CGI_VARIABLES.

Also expand the CGI_VARIABLE name listing to multiple lines for cleaner diffs and legibility.
Merge pull request #14971 from versioncontrol/#14785

Baseclass becomes! subclass
docs, update `includes` + `where` example in AR guide. [ci skip]

Closes #15541.
`bin/rake db:migrate:status` works with legacy migration numbers.

The migration numbers were normalized different ways. This left
the task output in an inconsistent state.

Closes #15538.