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.date'
    _inherit = 'ir.qweb.field'
    def value_to_html(self, cr, uid, value, field, options=None, context=None):
        if not value or len(value)<10: return ''
        lang = self.user_lang(cr, uid, context=context)
        locale = babel.Locale.parse(lang.code)
        if isinstance(value, basestring):
            value = datetime.datetime.strptime(
                value[:10], openerp.tools.DEFAULT_SERVER_DATE_FORMAT)
        if options and 'format' in options:
            pattern = options['format']
        else:
            strftime_pattern = lang.date_format
            pattern = openerp.tools.posix_to_ldml(strftime_pattern, locale=locale)
        return babel.dates.format_date(
            value, format=pattern,
            locale=locale)
class DateTimeConverter(osv.AbstractModel):