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)


for jscript in self.javascripts:
                    response.append(jscript.to_html())
        else:
            if qwebcontext is None:
                qwebcontext = QWebContext(self.cr, self.uid, {})
            if css and self.stylesheets:
                css_attachments = self.css()
                if not self.css_errors:
                    for attachment in css_attachments:
                        el = etree.fromstring('<link href="%s" rel="stylesheet"/>' % attachment.url)
                        response.append(self.registry['ir.qweb'].render_node(el, qwebcontext))
                else:
                    msg = '\n'.join(self.css_errors)
                    self.stylesheets.append(StylesheetAsset(self, inline=self.css_message(msg)))
                    for style in self.stylesheets:
                        response.append(style.to_html())
            if js and self.javascripts:
                el = etree.fromstring('<script %s type="text/javascript" src="%s"></script>' % (async and 'async="async"' or '', self.js().url))
                response.append(self.registry['ir.qweb'].render_node(el, qwebcontext))
        response.extend(self.remains)
        return sep + sep.join(response)
    @lazy_property
    def last_modified(self):
        """Returns last modified date of linked files"""
        return max(itertools.chain(
            (asset.last_modified for asset in self.javascripts),
            (asset.last_modified for asset in self.stylesheets),
        ))
    @lazy_property
    def version(self):