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
if self.eval_bool(template_attributes["if"], qwebcontext):
            return self.render_element(element, template_attributes, generated_attributes, qwebcontext)
        return ""
    def render_tag_call(self, element, template_attributes, generated_attributes, qwebcontext):
        d = qwebcontext.copy()
        if 'lang' in template_attributes:
            init_lang = d.context.get('lang', 'en_US')
            lang = template_attributes['lang']
            d.context['lang'] = self.eval(lang, d) or lang
            if not self.pool['res.lang'].search(d.cr, d.uid, [('code', '=', d.context['lang'])], count=True, context=d.context):
                _logger.info("'%s' is not a valid language code, is an empty field or is not installed, falling back to en_US", lang)
        d[0] = self.render_element(element, template_attributes, generated_attributes, d)
        cr = d.get('request') and d['request'].cr or None
        uid = d.get('request') and d['request'].uid or None
        template = self.eval_format(template_attributes["call"], d)
        try:
            template = int(template)
        except ValueError:
            pass
        d['generated_attributes'] = generated_attributes
        res = self.render(cr, uid, template, d)
        
        if 'lang' in template_attributes:
            d.context['lang'] = init_lang
        return res
    def render_tag_call_assets(self, element, template_attributes, generated_attributes, qwebcontext):