Fix 'method redefined' warning.

Running the following test was throwing the below warnings
> cd railties
> ruby -w -Itest test/generators/named_base_test.rb

    test/generators/named_base_test.rb:9: warning: method redefined;
    discarding old pluralize_table_names
    ../activesupport/lib/active_support/core_ext/class/attribute.rb:86:
    warning: previous definition of pluralize_table_names was here

    test/generators/named_base_test.rb:9: warning: method redefined;
    discarding old pluralize_table_names=
    ../activesupport/lib/active_support/core_ext/class/attribute.rb:83:
    warning: previous definition of pluralize_table_names= was here