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)


else:
                        self.stylesheets.append(StylesheetAsset(self, url=href, media=media))
                elif el.tag == 'script' and not src:
                    self.javascripts.append(JavascriptAsset(self, inline=el.text))
                elif el.tag == 'script' and self.can_aggregate(src):
                    self.javascripts.append(JavascriptAsset(self, url=src))
                else:
                    self.remains.append(html.tostring(el))
            else:
                try:
                    self.remains.append(html.tostring(el))
                except Exception:
                    
                    raise NotImplementedError
    def can_aggregate(self, url):
        return not urlparse(url).netloc and not url.startswith('/web/content')
    def to_html(self, sep=None, css=True, js=True, debug=False, async=False, qwebcontext=None):
        if sep is None:
            sep = '\n            '
        response = []
        if debug:
            if css and self.stylesheets:
                if not self.is_css_preprocessed():
                    self.preprocess_css(debug=debug)
                    if self.css_errors:
                        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: