take into account line_items with negative quantity

[SubandiK]

This makes it so that secretariat accepts an invoice like this:

• invoice total: -100€
• invoice item: 1x -100€

and then turning the invoice item to "-1x 100€", so the resulting e-invoice passes validations.

Also added a small change: invoice_items with an amount of 0 but a non 0 tax rate, e.g.
title: booking, amount: 0€, tax_rate: 19%, tax_amount: 0€
will be sent as having a tax_rate of 0%, otherwise they don't get accepted by the standard as is.

fix #21

Background: Zugferd doesn't allow invoice items with negative amounts; it has no problems with negative amounts for invoices. By accepting an invoice with attributes like "1x -100€" and then turning them into "-1x 100€" we stay standard conformant.

I decided to do it like this, and not make this gem's api expect the "-1x 100€" format because I think this is more straight forward and what most people using this gem will already use internally. But I can switch it around and put the burden on the gem-user if you prefer.

Can be reviewed @halfbyte :)