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)


try:
            return qwebcontext.safe_eval(expr)
        except Exception:
            template = qwebcontext.get('__template__')
            raise_qweb_exception(message="Could not evaluate expression %r" % expr, expression=expr, template=template)
    def eval_object(self, expr, qwebcontext):
        return self.eval(expr, qwebcontext)
    def eval_str(self, expr, qwebcontext):
        if expr == "0":
            return qwebcontext.get(0, '')
        val = self.eval(expr, qwebcontext)
        if isinstance(val, unicode):
            return val.encode("utf8")
        if val is False or val is None:
            return ''
        return str(val)
    def eval_format(self, expr, qwebcontext):
        expr, replacements = self._format_regex.subn(
            lambda m: self.eval_str(m.group(1) or m.group(2), qwebcontext),
            expr
        )
        if replacements:
            return expr
        try:
            return str(expr % qwebcontext)
        except Exception:
            template = qwebcontext.get('__template__')
            raise_qweb_exception(message="Format error for expression %r" % expr, expression=expr, template=template)
    def eval_bool(self, expr, qwebcontext):