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
content = self.rx_sourceMap.sub('', self.content)
        
        content = re.sub(r'/\*.*?\*/', '', content, flags=re.S)
        
        content = re.sub(r'\s+', ' ', content)
        content = re.sub(r' *([{}]) *', r'\1', content)
        return self.with_header(content)
    def to_html(self):
        media = (' media="%s"' % werkzeug.utils.escape(self.media)) if self.media else ''
        if self.url:
            href = self.html_url
            return '<link rel="stylesheet" href="%s" type="text/css"%s/>' % (href, media)
        else:
            return '<style type="text/css"%s>%s</style>' % (media, self.with_header())
class PreprocessedCSS(StylesheetAsset):
    rx_import = None
    def __init__(self, *args, **kw):
        super(PreprocessedCSS, self).__init__(*args, **kw)
        self.html_url_format = '%%s/%s/%%s.css' % self.bundle.xmlid
        self.html_url_args = tuple(self.url.rsplit('/', 1))
    def get_source(self):
        content = self.inline or self._fetch_content()
        return "/*! %s */\n%s" % (self.id, content)
    def get_command(self):
        raise NotImplementedError
class SassStylesheetAsset(PreprocessedCSS):