Added documentation for css_compressor in asset_pipeline.md

Log for which keys values were set to nil in deep_munge

Handle aliased attributes in AR::Relation#select, #order, etc.

With this we can write `Model#select(:aliased)`, `Model#order(:aliased)`,
`Model#reoder(aliased: :desc)`, etc.

Supplementary work to 54122067.

deep_munge solves CVE-2013-0155 security vulnerability, but its
behaviour is definately confuisng. This commit adds logging to deep_munge.
It logs keys for which values were set to nil.

Also mentions in guides were added.

Add docs about behaviour of replacing a has_one associate object.

Add docs about behaviour of replacing a has_one associate object, that the previous one is deleted even if the new one
 doesn't get persisted to database.

 Fixes #13197 . [ci skip]

Replace File.exists? with File.exist?

To quell warnings on ruby 2.1

use the new clear_validators! api everywhere to reset validators in tests

Rails config for raise on missing translations

The request attributes filtered_parameters, filtered_env and filtered_path
are memoized for performance reasons. However this can cause unusual
behavior in tests where there are multiple calls to get, post, etc.

Fixes #13803.

Ability to specify multiple contexts when defining a validation.

Add a config to setup whether raise exception for missing translation or
not.

Display value when raising due to unscope() issues

Example:

validates_presence_of :name, on: [:update, :custom_validation_context]

Ruby on Rails 4.1 Release Notes: Fix spelling [ci skip]

Hopefully make it easier to debug errors. e.g

Before:

    RuntimeError:
       unscope(where: "deleted_at") failed: unscoping String is unimplemented.

After:

    RuntimeError:
       unscope(where: "deleted_at") failed: unscoping String "'t'='t'" is unimplemented.

Both Time#xmlschema and DateTime#xmlschema can accept nil values for the
fraction_digits parameter. This commit makes this so for TimeWithZone
values as well.

The subsecond fraction digits had been hardcoded to 3. This forced all
timestamps to include the subsecond digits with no way to customize the
value. While the subsecond format is part of the ISO8601 spec, it is not
adhered to by all parsers (notably mobile clients). This adds the
ability to customize the number of digits used, optionally setting them
to 0 in order to eliminate the subsecond fraction entirely:

ActiveSupport::JSON::Encoding.subsecond_fraction_digits = 0

Modern RDoc accepts a few more things in +...+. In particular symbols work now.

The current regexp in RDoc is https://github.com/rdoc/rdoc/blob/v4.1.1/lib/rdoc/markup/attribute_manager.rb#L133.

Closes #11247.

Fix order syntax in find_by_sql example [ci skip]