Use default charset/collation only if *neither* is specified. Otherwise we set utf8_unicode_ci collation on latin1 table.

Conflicts:
	actionmailer/lib/action_mailer/base.rb
	activesupport/lib/active_support/configurable.rb
	activesupport/lib/active_support/core_ext/module/deprecation.rb
	guides/source/action_controller_overview.md
	guides/source/active_support_core_extensions.md
	guides/source/ajax_on_rails.textile
	guides/source/association_basics.textile
	guides/source/upgrading_ruby_on_rails.md

While resolving conflicts, I have chosen to ignore changes done in
docrails at some places - these will be most likely 1.9 hash syntax
changes.

It's too hard to test this properly, so let's just check that there are
no errors.

Fix MIME type in CHANGELOG [ci skip]

If your database supports setting the isolation level for a transaction,
you can set it like so:

  Post.transaction(isolation: :serializable) do
    # ...
  end

Valid isolation levels are:

* `:read_uncommitted`
* `:read_committed`
* `:repeatable_read`
* `:serializable`

You should consult the documentation for your database to understand the
semantics of these different levels:

* http://www.postgresql.org/docs/9.1/static/transaction-iso.html
* https://dev.mysql.com/doc/refman/5.0/en/set-transaction.html

An `ActiveRecord::TransactionIsolationError` will be raised if:

* The adapter does not support setting the isolation level
* You are joining an existing open transaction
* You are creating a nested (savepoint) transaction

The mysql, mysql2 and postgresql adapters support setting the
transaction isolation level. However, support is disabled for mysql
versions below 5, because they are affected by a bug
(http://bugs.mysql.com/bug.php?id=39170) which means the isolation level
gets persisted outside the transaction.

CHANGELOG edits do not belongs on docrails, so I'll send a pull request
instead. Thanks @rafaelfranca for catching this.

This reverts commit 84c9491e.

Callbacks may interfere with each other.

Merge pull request #5248 from jcoleman/should-unset-association-when-an-existing-record-is-destroyed

Unset association when existing record is destroyed.

See #7613. [ci skip]