Quick fix for ActiveModel error_wrapping

I found this bug when trying to use an ActiveMerchant credit card object in a form. ActiveMercahnt does it's own validations but there aren't keys for fields with no errors so it fails.

The simple fix is:

module ActiveModelInstanceTag
  def error_wrapping(html_tag)
    if object.respond_to?(:errors) && object.errors.respond_to?(:full_messages) && object.errors[@method_name] && object.errors[@method_name].any?
      Base.field_error_proc.call(html_tag, self)
    else
      html_tag
    end
  end
end

All I added was "&& object.errors[@method_name]" to check if the field exists in the errors array. Let me know if you'd like me to submit a patch. This is pretty simple.