Fix case sensitivity regression in date standard filter

The to_date refactor on Apr 30 removed downcase from the obj switch, this patch adds back a case-insensitive regexp to check for 'now' and 'today' as well as a test for it.
You could compare with

```ruby
case obj.downcase
when 'now'.freeze, 'today'.freeze
```

which is what we were doing before this regression was introduced.