Perf fix - Use an instance variable instead of a class_attribute. Thanks @josevalim and @jhawthorn for the prompting.

ActiveSupport::Dependencies now raises NameError if it finds an existing
constant in load_missing_constant. This better reflects the nature of
the error which is usually caused by calling constantize on a nested constant.

Closes #1423

Add ActiveRecord::attribute_names to retrieve a list of attribute names. This method will also return an empty array on an abstract class or a model that the table doesn't exists.

This reverts commit 886818d2.

Revert "Merge pull request #423 from richardiux/master"

This reverts commit b8f08c4e, reversing
changes made to fd9df1b1.

Adding base method symbolized_sti_name to activerecord base to be used on identity map. Identity map now considers the inheritance when creating the caching keys

deprecated the use of the guard_protected_attributes argument with attributes= in AR in favor of assign_attributes(attrs, :without_protection => true)

Added assign_attributes to Active Record which accepts a mass-assignment security scope using the :as option, while also allowing mass-assignment security to be bypassed using :with_protected

Bring back support for passing a callable object to the default_scope macro. You can also just use a block.

Un-deprecate using 'default_scope' as a macro, but if you are calling the macro multiple times that will give deprecation warnings, and in 3.2 we will simply overwrite the default scope when you call the macro multiple times.

Evaluate default scopes at the last possible moment in order to avoid problems with default scopes getting included into other scopes and then being unable to remove the default part via unscoped.

Deprecated support for passing hashes and relations to default_scope, in favour of defining a 'default_scope' class method in the model. See the CHANGELOG for more details.