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)


elif trim == 'both':
            inner = inner.strip()
        if name == "t":
            return inner
        elif len(inner) or name not in self._void_elements:
            return "<%s%s>%s</%s>" % tuple(
                qwebcontext if isinstance(qwebcontext, str) else qwebcontext.encode('utf-8')
                for qwebcontext in (name, generated_attributes, inner, name)
            )
        else:
            return "<%s%s/>" % (name.encode("utf-8"), generated_attributes)
    def render_attribute(self, element, name, value, qwebcontext):
        return _build_attribute(name, value)
    def render_text(self, text, element, qwebcontext):
        return text.encode('utf-8')
    def render_tail(self, tail, element, qwebcontext):
        return tail.encode('utf-8')
    
    def render_att_att(self, element, attribute_name, attribute_value, qwebcontext):
        if attribute_name.startswith("t-attf-"):
            return [(attribute_name[7:], self.eval_format(attribute_value, qwebcontext))]
        if attribute_name.startswith("t-att-"):
            return [(attribute_name[6:], self.eval(attribute_value, qwebcontext))]
        result = self.eval_object(attribute_value, qwebcontext)
        if isinstance(result, collections.Mapping):
            return result.iteritems()
        
        return [result]
    
    def render_tag_raw(self, element, template_attributes, generated_attributes, qwebcontext):