`.is_a?` works with constant values

## Bug Report

This came up in the Discord and was surprised this even works:

```cr
require "http/status"

pp HTTP::Status::OK.is_a? HTTP::Status::OK        # => true
pp HTTP::Status::NOT_FOUND.is_a? HTTP::Status::OK # => false
```

Or more simply:

```cr
NUM = 1
 
pp NUM.is_a? NUM # => true
```

Is this expected?