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)


page = []
            pages = [page]
            page_selectors = 0
            for rule in re.findall(re_rules, css):
                selectors = len(re.findall(re_selectors, rule))
                if page_selectors + selectors <= self.max_css_rules:
                    page_selectors += selectors
                    page.append(rule)
                else:
                    pages.append([rule])
                    page = pages[-1]
                    page_selectors = selectors
            for idx, page in enumerate(pages):
                self.save_attachment("css", ' '.join(page), inc=idx)
            attachments = self.get_attachments('css')
        return attachments
    def css_message(self, message):
        
        message = message.replace('\n', '\\A ').replace('"', '\\"')
        return """
            body:before {
                background: #ffc;
                width: 100%%;
                font-size: 14px;
                font-family: monospace;
                white-space: pre;
                content: "%s";
            }
        """ % message
    def is_css_preprocessed(self):