message quote fix

Since ActiveModel::Errors instance keeps all error messages as hash
we should duplicate this object as well.

Previously ActiveModel::Errors was a subclass of ActiveSupport::OrderedHash,
which results in different behavior on `dup`, this may result in regression for
people relying on it.

Without that patch when using ActiveModel::AttributeMethods
in a class that does not respond to `attributes` method,
stack level too deep error will be raised on non existing
method. While documentation is clear that you need to define
`attributes` method in order to use AttributeMethods module,
`stack level too deep` is rather obscure and hard to debug,
therefore we should try to not break `method_missing` if
someone forgets about defining `attributes`.

Fixed failing tests
This reverts commit 8350ce97.

This reverts commit 675094b2.

Fix grammatical error in ActiveModel::MassAssignmentSecurity::PermissionSet#deny? NotImplementedError message

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