Fix bug introduced by changeset 3679 which caused custom attribute? methods to be overridden. Also ensure that ? methods are defined even if read method is customised. (closes #3677) [jonathan@bluewire.net.nz]

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@4002 5ecf4fe2-1ee6-0310-87b1-e25e094e27de