Month Formatting Bug

The %B date formatting operator relies on nil for the first month in the month_names array. Deleting it from I18n will cause month shifting.

To reproduce the issue:

```ruby
I18n.l(Time.now, :format => "%B %d, %Y")
I18n.t(:month_names, :scope => :date).delete_if(&:blank?)
I18n.l(Time.now, :format => "%B %d, %Y")
```