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
uid = qwebcontext.get('request') and qwebcontext['request'].uid or None
                can_see = self.user_has_groups(cr, uid, groups=attribute_value) if cr and uid else False
                if not can_see:
                    return element.tail and self.render_tail(element.tail, element, qwebcontext) or ''
            attribute_value = attribute_value.encode("utf8")
            if attribute_name.startswith("t-"):
                for attribute in self._render_att:
                    if attribute_name[2:].startswith(attribute):
                        attrs = self._render_att[attribute](
                            self, element, attribute_name, attribute_value, qwebcontext)
                        for att, val in attrs:
                            if not val: continue
                            generated_attributes += self.render_attribute(element, att, val, qwebcontext)
                        break
                else:
                    if attribute_name[2:] in self._render_tag:
                        t_render = attribute_name[2:]
                    template_attributes[attribute_name[2:]] = attribute_value
            else:
                generated_attributes += self.render_attribute(element, attribute_name, attribute_value, qwebcontext)
        if t_render:
            result = self._render_tag[t_render](self, element, template_attributes, generated_attributes, qwebcontext)
        else:
            result = self.render_element(element, template_attributes, generated_attributes, qwebcontext)
        if element.tail:
            result += self.render_tail(element.tail, element, qwebcontext)
        if isinstance(result, unicode):
            return result.encode('utf-8')
        return result
    def render_element(self, element, template_attributes, generated_attributes, qwebcontext, inner=None):