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)


g_att += ''.join(
                _build_attribute(name, value)
                for name, value in self.attributes(
                    cr, uid, field_name, record, options,
                    source_element, g_att, t_att, qweb_context,
                    context=context)
            )
        return self.render_element(cr, uid, source_element, t_att, g_att,
                                   qweb_context, content)
    def qweb_object(self):
        return self.pool['ir.qweb']
    def render_element(self, cr, uid, source_element, t_att, g_att,
                       qweb_context, content):
        """ render_element(cr, uid, source_element, t_att, g_att, qweb_context, content)
        Final rendering hook, by default just calls ir.qweb's ``render_element``
        """
        return self.qweb_object().render_element(
            source_element, t_att, g_att, qweb_context, content or '')
    def user_lang(self, cr, uid, context):
        """ user_lang(cr, uid, context)
        Fetches the res.lang object corresponding to the language code stored
        in the user's context. Fallbacks to en_US if no lang is present in the
        context *or the language code is not valid*.
        :returns: res.lang browse_record
        """
        if context is None: context = {}
        lang_code = context.get('lang') or 'en_US'
        Lang = self.pool['res.lang']
        return Lang.browse(cr, uid, Lang._lang_get(cr, uid, lang_code), context=context)
class IntegerConverter(osv.AbstractModel):