Support any args in NilClass#to_s

Hi,

[Rails extends DateTime to receive one argument](https://github.com/rails/rails/blob/v4.1.6.rc2/activesupport/lib/active_support/core_ext/date_time/conversions.rb#L41) so I think we should extend NilClass to avoid the following error.

```ruby
> user = User.new
> user.deleted_at.to_s :db
ArgumentError: wrong number of arguments (1 for 0)
```

Datetime values of ActiveRecord can often be `nil`, but everyone forget it then call this method in this way.

My current workaround is here.

```ruby
> user = User.new
> user.deleted_at.try(:to_s, :db)
nil
```

However, it doesn't make sense that we wrap it with try everywhere.