There is an example in Rails documentation that suggests implementing
assign_attributes method for ActiveModel interface, that by default
sends option role with nil. Since mass_assignment_authorizer never
is called without args, we can move the default value internally.

The problem: We need to be able to specify configuration in a way that
can be inherited to models that include ActiveRecord::Model. So it is
no longer sufficient to put 'top level' config on ActiveRecord::Base,
but we do want configuration specified on ActiveRecord::Base and
descendants to continue to work.

So we need something like class_attribute that can be defined on a
module but that is inherited when ActiveRecord::Model is included.

The solution: added ActiveModel::Configuration module which provides a
config_attribute macro. It's a bit specific hence I am not putting this
in Active Support or making it a 'public API' at present.

Problems with current example:

* DOESN'T WORK 
** attr_protected :last_login, :as => :admin # doesn't make it accessible for admin
* Uses ActiveSupport Fixnum extension

In order to specify your own sanitize method
Implemented .mass_assignment_sanitizer configuration option

Added an ability to specify your own behavior on mass assingment
protection, controlled by option:
ActiveModel::MassAssignmentSecurity.mass_assignment_sanitizer

AM mass assignment security attr_accessible and attr_protected now allow for scopes using :as => scope eg.
    
    attr_accessible :name
    attr_accessible :name, :admin, :as => :admin

Change documentation for ActiveModel::MassAssignmentSecurity a bit and make debug always be called since some people may overwrite warn! to add extra behavior even if logger is not available.

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Signed-off-by: NJosé Valim <jose.valim@gmail.com>