Add #nodoc to initialize_dup and use :method: to document the #dup method.
Relates to issue #6235

The #relation method gets called in four places and the return value was instantly cloned in three of them. The only place that did not clone was ActiveRecord::Scoping::Default::ClassMethods#unscoped. This introduced a bug described in #5667 and should really clone the relation, too. This means all four places would clone the relation, so it doesn't make a lot of sense caching it in the first place.

The four places with calls to relations are:

activerecord/lib/active_record/scoping/default.rb:110:in `block in build_default_scope'"
activerecord/lib/active_record/scoping/default.rb:42:in `unscoped'"
activerecord/lib/active_record/scoping/named.rb:38:in `scoped'"
activerecord/lib/active_record/scoping/named.rb:52:in `scope_attributes'"

This apparently fix the warning related to @new_record variable not
being initialized in AR's test suit, when an association is built and
the object is marshalled/loaded.

See these tests in AR's base_test.rb:

test_marshalling_with_associations
test_marshalling_new_record_round_trip_with_associations

Closes #3720.

copy the columns hash to the active record instances, typecast using columns looked up on the instance

another value has been specified).

From 2c667f69.

Thanks @pwnall for the heads-up.

This is the 'top level' connection, inherited by any models that include
ActiveRecord::Model or inherit from ActiveRecord::Base.

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.