Impacted versions: Odoo 11 on windows server 2012

Steps to reproduce: 
Create a new out invoice, select a product and set a negative amount for unit price.
Print the invoice.

Current behavior:
in PDF the char "-" is shown as "|" even if the total amount is correct (amount1 - amount2 = total)

Expected behavior:
the printed invoice should show the char "-"

Video/Screenshot link (optional):
![image](https://user-images.githubusercontent.com/37513316/42877323-8c712b2e-8a89-11e8-9690-27f00e08b6b2.png)

this is the negative item on odoo:
![image](https://user-images.githubusercontent.com/37513316/42877389-bf04eba2-8a89-11e8-91c0-e38059673b42.png)


raise NotImplementedError()
    @lazy_property
    def last_modified(self):
        try:
            self.stat()
            if self._filename:
                return datetime.datetime.fromtimestamp(os.path.getmtime(self._filename))
            elif self._ir_attach:
                server_format = openerp.tools.misc.DEFAULT_SERVER_DATETIME_FORMAT
                last_update = self._ir_attach['__last_update']
                try:
                    return datetime.datetime.strptime(last_update, server_format + '.%f')
                except ValueError:
                    return datetime.datetime.strptime(last_update, server_format)
        except Exception:
            pass
        return datetime.datetime(1970, 1, 1)
    @property
    def content(self):