[12.x] Fix adding invoice item with quantities

[driesvints]

Apparently if you use a quantity here you're required to use unit_amount instead of amount. No idea why no-one ever stumbled on this before.

Fixes #1153

[PHLAK]

FYI, this (minor) change broke our app after a routine dependency update. Before this change we were doing the following

$account->tab($description, null, [
    'unit_amount' => $unitAmount,
    'quantity' => $quantity,
]);

This resulted in amount being null and unit_amount and quantity being set in the request as expected.

However, after this change unit_amount gets overridden with the value of $amount (null) and resulted in neither amount nor unit_amount being set in the request. That in turn resulted in the same error mentioned in #1153 that this PR was supposed to fix.

I'm not saying this PR is bad or wrong (we may have been misusing this method to begin with), I just wanted to shed some light in case someone else runs into the same issue.

[driesvints]

@PHLAK thanks for reporting that. Definitely a use case I wasn't thinking of. I tagged v12.14.1 with a fix for that.

You could just upgrade to the following now:

$account->tab($description, $unitAmount, [
    'quantity' => $quantity,
]);

[PHLAK]

Thanks @driesvints, this is what we're doing now.