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.html'
    _inherit = 'ir.qweb.field'
    def value_to_html(self, cr, uid, value, field, options=None, context=None):
        return HTMLSafe(value or '')
class ImageConverter(osv.AbstractModel):
    """ ``image`` widget rendering, inserts a data:uri-using image tag in the
    document. May be overridden by e.g. the website module to generate links
    instead.
    .. todo:: what happens if different output need different converters? e.g.
              reports may need embedded images or FS links whereas website
              needs website-aware
    """
    _name = 'ir.qweb.field.image'
    _inherit = 'ir.qweb.field'
    def value_to_html(self, cr, uid, value, field, options=None, context=None):
        try:
            image = Image.open(cStringIO.StringIO(value.decode('base64')))
            image.verify()
        except IOError:
            raise ValueError("Non-image binary fields can not be converted to HTML")
        except: 
            raise ValueError("Invalid image content")
        return HTMLSafe('<img src="data:%s;base64,%s">' % (Image.MIME[image.format], value))
class MonetaryConverter(osv.AbstractModel):