minorbug(date): using "+%_::::z" produces different output across the two versions, gnu vs uu output

Hi, uutils mainteners

that is a minor bug, our test finded

```css
relunsec@relunsec:~/software/coreutils/target/debug$ ./date "+%_::::z" 
%::::z
relunsec@relunsec:~/software/coreutils/target/debug$ gnudate "+%_::::z" 
%_::::z
relunsec@relunsec:~/software/coreutils/target/debug$ 
```

as i see the gnu one keeps the _ while the uutils one strips it, that is a difference in the output