Impacted versions: ODOO 10

Problem.
If i print a invoice with a negative amount, the pdf shows a vertical bar instead of a minus symbol.
![image](https://cloud.githubusercontent.com/assets/28800797/26237253/b07fadb0-3c74-11e7-86c2-c8688d82b5a5.png)

I have tried to install a new pdf reader, but with no result.
openerp/addons/base/ir/ir_qweb.py
units = dict(TIMEDELTA_UNITS)
        if value < 0:
            raise ValueError(_("Durations can't be negative"))
        if not options or options.get('unit') not in units:
            raise ValueError(_("A unit must be provided to duration widgets"))
        locale = babel.Locale.parse(
            self.user_lang(cr, uid, context=context).code)
        factor = units[options['unit']]
        sections = []
        r = value * factor
        if options.get('round') in units:
            round_to = units[options['round']]
            r = round(r / round_to) * round_to
        for unit, secs_per_unit in TIMEDELTA_UNITS:
            v, r = divmod(r, secs_per_unit)
            if not v: continue
            section = babel.dates.format_timedelta(
                v*secs_per_unit, threshold=1, locale=locale)
            if section:
                sections.append(section)
        return ' '.join(sections)
class RelativeDatetimeConverter(osv.AbstractModel):