Breaking change related to short options in 6.0.0?

Hey,

Previously the following:

```
commander
    .command('logs')
    .option("-n <nb-lines>", "Number of lines")
    .description('Print logs')
    .action(async function(opts) {
      ....

```

The option n was populated in **opts.N** (note the capital letter), but now it is no more available opts.N but is now available in **opts.n**. Is it a bug or the correct new behaviour?

