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
_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):