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.
_name = 'ir.qweb.field.relative'
    _inherit = 'ir.qweb.field'
    def value_to_html(self, cr, uid, value, field, options=None, context=None):
        parse_format = openerp.tools.DEFAULT_SERVER_DATETIME_FORMAT
        locale = babel.Locale.parse(
            self.user_lang(cr, uid, context=context).code)
        if isinstance(value, basestring):
            value = datetime.datetime.strptime(value, parse_format)
        
        reference = datetime.datetime.strptime(field.now(), parse_format)
        return babel.dates.format_timedelta(
            value - reference, add_direction=True, locale=locale)
class Contact(orm.AbstractModel):