Clarify the documentation of the primary_key option for associations

Previously the documentation stated that `primary_key` should be the name of
a *method* that returns the primary key used for the association. This is
incorrect. This changes the documentation to say that the value must be the
name of a column.
Merge pull request #17012 from petedmarsh/clarify-association-primary-key-option-documentation

Clarify the documentation of the primary_key option for associations
add a truncate method to the connection

it doesn't work on SQLite3 since it doesn't support truncate, but that's
OK.  If you call truncate on the connection, you're now bound to that
database (same as if you use hstore or any other db specific feature).
add message to `assert` in `assert_enqueued_with`
Update changelog with tenderlove's entry

[ci skip]
Merge pull request #17018 from prathamesh-sonpatki/tenderlove

Update changelog with tenderlove's entry
ar/connection_pool: honor overriden rack.test in middleware

Honoring an overidden `rack.test` allows testing closed connection between
multiple requests. This is useful if you're working on database resiliency, to
ensure the connection is in the expected state from one request to another on
the same worker.
Added test for exception message for validate method

- Test case for https://github.com/rails/rails/pull/16851
Merge pull request #17022 from prathamesh-sonpatki/test-for-validate-message

Added test for exception message for validate method
Merge pull request #17005 from y-yagi/fix_aj_test_helper

add message to `assert` in `assert_enqueued_with`
Added note about updating pull requests by force push

- Added note about force pushing same branch to update a existing pull
  request instead of opening new one in contributing guide.
- [ci skip]
Fix loading fixtures in engine tests

fixture_path is a class attribute of ActiveSupport::TestCase, so
ActiveSupport::TestCase.method_defined?(:fixture_path=) would always
return false.

This should fix #4971
Merge pull request #17026 from prathamesh-sonpatki/mention-updating-pull-requests

Added note about updating pull requests by force push
Merge pull request #17029 from grk/fix-loading-fixtures-in-engine-tests

Fix loading fixtures in engine tests
[CI SKIP] Improve force push note.
Merge pull request #17032 from tgxworld/update_contributing_guide

[CI SKIP] Improve force push note.
Merge pull request #17020 from Sirupsen/ar-override-rack-test

activerecord/connection_pool: honor overidden rack.test in tests