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
""" returns a :class:`~openerp.models.Model` used to render a
        ``t-field``.
        By default, tries to get the model named
        :samp:`ir.qweb.field.{field_type}`, falling back on ``ir.qweb.field``.
        :param str field_type: type or widget of field to render
        """
        return self.pool.get('ir.qweb.field.' + field_type, self.pool['ir.qweb.field'])
    def get_widget_for(self, widget):
        """ returns a :class:`~openerp.models.Model` used to render a
        ``t-esc``
        :param str widget: name of the widget to use, or ``None``
        """
        widget_model = ('ir.qweb.widget.' + widget) if widget else 'ir.qweb.widget'
        return self.pool.get(widget_model) or self.pool['ir.qweb.widget']
    def get_attr_bool(self, attr, default=False):
        if attr:
            attr = attr.lower()
            if attr in ('false', '0'):
                return False
            elif attr in ('true', '1'):
                return True
        return default
class FieldConverter(osv.AbstractModel):