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
def minify(self):
        return self.with_header(rjsmin(self.content))
    def _fetch_content(self):
        try:
            return super(JavascriptAsset, self)._fetch_content()
        except AssetError, e:
            return "console.error(%s);" % json.dumps(e.message)
    def to_html(self):
        if self.url:
            return '<script type="text/javascript" src="%s"></script>' % (self.html_url)
        else:
            return '<script type="text/javascript" charset="utf-8">%s</script>' % self.with_header()
class StylesheetAsset(WebAsset):
    rx_import = re.compile(r"""@import\s+('|")(?!'|"|/|https?://)""", re.U)
    rx_url = re.compile(r"""url\s*\(\s*('|"|)(?!'|"|/|https?://|data:)""", re.U)
    rx_sourceMap = re.compile(r'(/\*# sourceMappingURL=.*)', re.U)
    rx_charset = re.compile(r'(@charset "[^"]+";)', re.U)
    def __init__(self, *args, **kw):