Remove deprecated ActiveModel tests (%d and %s is no longer supported in error messages a couple months already)

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Be sure to convert namespaced names to we have 'Parrots name' instead of 'Parrots.name' in error messages.

Remove the test which was testing a non-existing method, always failing on 1.9 and never even running on 1.8

hash/conversions.rb needs active_support/core_ext/string/inflections because it uses singularize and a few others