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

Fixes #16578

Standardize on `Rails.application` [ci skip]

[ci skip] Fix typo in activerecord/test/cases/attribute_methods_test.rb:737

[ci skip] modify the description of production log level default

ActiveRecord CHANGELOG improvements [skip ci]

- Improve the error message by suggesting that the user may have
  intended to call validates instead of validate method.

They're valuable, but the types of errors they catch are fairly rare: we
can address them post-merge when they show up.

The hash is now string-keyed, and [_]reflect_on_association calls `to_s` on the
argument anyway.

For now, we will just skip the cache when a non-column key is used in the hash.
If the future, we can probably move some of the logic in PredicateBuilder.expand
up the chain to make caching possible for association queries.

Closes #16903
Fixes #16884

This is pretty lame though, so feel free to send a PR with a better fix! ;)

cc @zev @tenderlove

Update RecordNotFound exception cases to include a message with the