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.
return self.qweb_object().eval_object(
            currency, options['_qweb_context'])
TIMEDELTA_UNITS = (
    ('year',   3600 * 24 * 365),
    ('month',  3600 * 24 * 30),
    ('week',   3600 * 24 * 7),
    ('day',    3600 * 24),
    ('hour',   3600),
    ('minute', 60),
    ('second', 1)
)
class DurationConverter(osv.AbstractModel):
    """ ``duration`` converter, to display integral or fractional values as
    human-readable time spans (e.g. 1.5 as "1 hour 30 minutes").
    Can be used on any numerical field.
    Has a mandatory option ``unit`` which can be one of ``second``, ``minute``,
    ``hour``, ``day``, ``week`` or ``year``, used to interpret the numerical
    field value before converting it.
    Sub-second values will be ignored.
    """
    _name = 'ir.qweb.field.duration'
    _inherit = 'ir.qweb.field'
    def value_to_html(self, cr, uid, value, field, options=None, context=None):