Make update_attribute behave as in Rails 2.3 and document the behavior intrinsic to its implementation.

Signed-off-by: Nwycats <wycats@gmail.com>

layout_for works again with objects as specified in the documentation and Rails 2.3 [#5357 state:resolved]

Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

ActiveModel::Base doesn't exist, include_root_in_json is a class_attribute for the classes which includes this module

Revert "require_dependency should require using the normal mechanism if possible to avoid double-requires"

This was causing double requires since 991cd59a was reverted.
This reverts commit 8bf79739.