activerecord/connection_pool: honor overidden rack.test in tests

[CI SKIP] Improve force push note.

Fix loading fixtures in engine tests

Added note about updating pull requests by force push

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

This should fix #4971

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

add message to `assert` in `assert_enqueued_with`

Added test for exception message for validate method

- Test case for https://github.com/rails/rails/pull/16851

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.

Update changelog with tenderlove's entry

[ci skip]

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).

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.

Cleaner queuing of jobs using Delayed Job

Conflicts:
	activejob/lib/active_job/queue_adapters/delayed_job_adapter.rb

Fixtures using Foreign key symbols

fix fixtures not loading if a belongs_to association is defined with a :foreign_key option that's a symbol

This reverts commit 07635a74, reversing
changes made to 1b5f61a0.

Reason: it's not ready 💣, see https://github.com/rails/rails/pull/16888#issuecomment-56400816

Update error message of Active Model's validate method

Skip isolated tests for PRs

Changes default render behavior from file to template.

[ci skip] Added RDoc for each Active Job adapter

ActionMailer: mark mail as called after instead of before processing it

Remove wrapping <div> in form helpers from doc [ci skip]

[ci skip]

89ff1f82 and 1de258e6 removed from the HTML generated by the form helpers
the <div> that was wrapping the field elements inside the <form>.

This commit updates the documentation of the methods to reflect the
two commits above.

Fix another false assertions

 * The assertions in AS::Duration don't actually assert.
 * The assertion in Railtie will pass even when `eager_load_namespaces`
   doesn't include `AppTemplate::Application` if `Rails.application` is
   truthy.

For more details, see here:

 * https://github.com/rails/rails/pull/16998
 * https://github.com/rails/rails/pull/17000

Fix false assertions in AR::TestCase::AttributeMethodsTest

This test has always been green because it uses "assert" and the first
argument is an truthy class/object.

Document that the default for `rails server -b` has changed