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
fragments = html.fragments_fromstring(self.html)
        for el in fragments:
            if isinstance(el, basestring):
                self.remains.append(el)
            elif isinstance(el, html.HtmlElement):
                src = el.get('src', '')
                href = el.get('href', '')
                atype = el.get('type')
                media = el.get('media')
                if el.tag == 'style':
                    if atype == 'text/sass' or src.endswith('.sass'):
                        self.stylesheets.append(SassStylesheetAsset(self, inline=el.text, media=media))
                    elif atype == 'text/less' or src.endswith('.less'):
                        self.stylesheets.append(LessStylesheetAsset(self, inline=el.text, media=media))
                    else:
                        self.stylesheets.append(StylesheetAsset(self, inline=el.text, media=media))
                elif el.tag == 'link' and el.get('rel') == 'stylesheet' and self.can_aggregate(href):
                    if href.endswith('.sass') or atype == 'text/sass':
                        self.stylesheets.append(SassStylesheetAsset(self, url=href, media=media))
                    elif href.endswith('.less') or atype == 'text/less':
                        self.stylesheets.append(LessStylesheetAsset(self, url=href, media=media))