Fix regression with context-dependent validations in ActiveResource

As reported in #348, model#valid? is called with one argument while
ActiveResource::Validations only defines #valid? without an optional
argument.

The interface of ActiveResouce::Validations#valid? is the same for
A'resource in Rails 3.2 and the one extracted into a gem.

Rails 3.2 version:
https://github.com/rails/rails/blob/3-2-stable/activeresource/lib/active_resource/validations.rb#L123

Gem version:
https://github.com/rails/activeresource/blob/master/lib/active_resource/validations.rb#L160