Validation fixes

This fixes an issue with the base BlockValidator in ActiveModel.  The BlockValidator is used through the helper method `validates_each`:

```ruby
validates_each :price_points do |record, attr, value|
  if value[:number_available].nil?
    record.errors.add(attr, 'Rats! It looks like you forgot to tell us how much inventory you would like loaded. If you do not want to load any inventory for this price point on this day, either remove this price or tell us a quantity of 0.')
  end
end
```