AR::Base#becomes should change errors base to new object

When using 'becomes' to transform the class of an object, the base instance variable of the related ActiveModel::Errors object should be the transformed object, to prevent exceptions on validation.

Related to https://github.com/rails/rails/pull/3438

AR::Base#becomes should change errors base to new object

When using 'becomes' to transform the class of an object, the base instance variable of the related ActiveModel::Errors object should be the transformed object, to prevent exceptions on validation.

Related to https://github.com/rails/rails/pull/3438
Merge branch 'change_errors_base' of github.com:NiDi/rails into change_errors_base