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)


openerp/addons/base/ir/ir_qweb.py
attachments = self.get_attachments('js')
        if not attachments:
            content = ';\n'.join(asset.minify() for asset in self.javascripts)
            return self.save_attachment('js', content)
        return attachments[0]
    def css(self):
        attachments = self.get_attachments('css')
        if not attachments:
            
            css = self.preprocess_css()
            if self.css_errors:
                return
            
            matches = []
            css = re.sub(self.rx_css_import, lambda matchobj: matches.append(matchobj.group(0)) and '', css)
            matches.append(css)
            css = u'\n'.join(matches)
            
            re_rules = '([^{]+\{(?:[^{}]|\{[^{}]*\})*\})'
            re_selectors = '()(?:\s*@media\s*[^{]*\{)?(?:\s*(?:[^,{]*(?:,|\{(?:[^}]*\}))))'