This reverts commit a3024f81.

REASON: Since warning doesn't exist

Split Validations and Callbacks into separate guides.

Two big features that are only barely related in the same guide. Seems
bad.

I did not check references to these guides yet, so some links may need
to be updated.

Gemfile source url for plugins updated

Move instantiation responsibilities from Inheritance to Persistence. Have Inheritance#discriminate_class_for_record handle STI lookup duties.

Added examples to current_page? method [ci skip]

augmented the Rails 4 guides with links back to Github

Allows you to do BaseClass.new(:type => "SubClass") as well as
parent.children.build(:type => "SubClass") or parent.build_child
to initialize an STI subclass. Ensures that the class name is a
valid class and that it is in the ancestors of the super class
that the association is expecting.

Add #seconds_until_end_of_day to DateTime and Time

[ci skip]

Fix typo in the Observers deprecation message.

Rather than just raising a NoMethodError when copying the config, this
commit adds a warning message until either the rails-observers gem is
installed or the relevant config options are removed.

They was extracted from a plugin.

See https://github.com/rails/rails-observers

[Rafael Mendonça França + Steve Klabnik]

Simplify String#mb_chars and fix documentation.

This behavior mattered under Ruby 1.8, but that doesn't matter now
that we don't support it.

In addition, we don't want to proxy the #class method. A test was added
to prevent against regressions.

This documentation has been out of date.

Ensure that associations have a symbol argument.

Fixes #7418.

Specify type of singular assication during serialization

When serialising a class, specify the type of any singular associations, if
necessary. Rails already correctly specifies the :type of any enumerable
association (e.g. a has_many association), but made no attempt to do so for
non-enumerables (e.g. a has_one association).
We must specify the :type of any STI association. A has_one
association to a class which uses single-table inheritance is an example of
this type of association.

Fixes #7471

Revert "rails gem has no lib directory to require"

This reverts commit 6bcbd604.

The default value for require_paths is ['lib'], and Rails does not
need to include any lib/ file to build the gem, so the previous
commit 'makes sense'. However, gems will not be built at all if
require_paths is empty (see https://gist.github.com/1065473), so
it's better to stick with the default, which does not have any
side-effects

This table is being used to verify if the :limit options is being
ignored for text and binary columns

Remove FIXME tag from abstract adapter test.

Use File.join to better integrate fixture_path in fixture_file_upload.

Cache digest opt out cleanup

Instead of deleting the skip_digest option flag, this changes the method to merely check the
key. This change is because of a discussion in this thread:

https://github.com/rails/rails/pull/8317

This commit also makes #fragment_name_with_digest private due to its
functionality being subsumed by #cache_fragment_name.