Fix missing optional arguments to win over values

[svoop]

See #126 (comment)

Optional arguments (either implic or with explicit required: false) should be accepted when missing even if a values list does not include nil as a value – at least as per the principle of least surprise.

argument :required_arg, required: true, values: %w(one two)
argument :optional_arg, required: false, values: %w(one two)

cmd             # ERROR
cmd one         # OKAY
cmd three       # ERROR 
cmd one two     # OKAY
cmd one three   # ERROR

Not sure how best to spec this, hope it's okay to having added a vacuum subcommand to the db example.

/cc @gustavothecoder

[katafrakt]

As for spec, I was able to reproduce the bug (on main) like this: 6822f94
Perhaps you can pull that in?

Otherwise, it looks like a great fix to me!

[svoop]

@katafrakt Sorry, dropped the ball here:

Perhaps you can pull that in?

Not sure what you mean by that, shall I just add your tests to this PR?

[katafrakt]

@svoop I forgot about this PR, apologies. I pushed the test now to your branch (it needed some additional tweaking). I think we are good to go now with this?

[katafrakt]

Let's add a changelog entry though, before merging.

[svoop]

@katafrakt Thanks a bunch!