Preserve literal semantics in strict2 case/when

Previously, strict2 case/when used `safe_parse_expression`
to parse when expressions causing `blank`/`empty` to be
treated as string literals (Expression::LITERALS maps 'empty' => ''),
rather than method literals

This caused unexpected behavior:

```
{%- case empty_obj -%}
{%- when empty -%}
  previously: doesn't render (empty_obj == '' is false)
  now: renders (empty_obj.empty? is true)
{%- endcase -%}
```

This commit instead calls `Condition.parse_expression`
with `safe: true`, which will correctly handle `blank`
and `empty`

lib/liquid/tags/case.rb

@@ -118,7 +118,7 @@ module Liquid
       parser = @parse_context.new_parser(markup)
 
       loop do
-        expr = safe_parse_expression(parser)
+        expr = Condition.parse_expression(parse_context, parser.expression, safe: true)
         block = Condition.new(@left, '==', expr)
         block.attach(body)
         @blocks << block