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)


self.media = kw.pop('media', None)
        super(StylesheetAsset, self).__init__(*args, **kw)
    @property
    def content(self):
        content = super(StylesheetAsset, self).content
        if self.media:
            content = '@media %s { %s }' % (self.media, content)
        return content
    def _fetch_content(self):
        try:
            content = super(StylesheetAsset, self)._fetch_content()
            web_dir = os.path.dirname(self.url)
            if self.rx_import:
                content = self.rx_import.sub(
                    r"""@import \1%s/""" % (web_dir,),
                    content,
                )
            if self.rx_url:
                content = self.rx_url.sub(
                    r"url(\1%s/" % (web_dir,),
                    content,
                )
            if self.rx_charset:
                
                content = self.rx_charset.sub('', content)
            return content
        except AssetError, e:
            self.bundle.css_errors.append(e.message)
            return ''
    def minify(self):