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)


_name = 'ir.qweb.field.contact'
    _inherit = 'ir.qweb.field.many2one'
    def record_to_html(self, cr, uid, field_name, record, options=None, context=None):
        if context is None:
            context = {}
        if options is None:
            options = {}
        opf = options.get('fields') or ["name", "address", "phone", "mobile", "fax", "email"]
        value_rec = record[field_name]
        if not value_rec:
            return None
        value_rec = value_rec.sudo().with_context(show_address=True)
        value = value_rec.name_get()[0][1]
        val = {
            'name': value.split("\n")[0],
            'address': escape("\n".join(value.split("\n")[1:])).strip(),
            'phone': value_rec.phone,
            'mobile': value_rec.mobile,
            'fax': value_rec.fax,
            'city': value_rec.city,
            'country_id': value_rec.country_id.display_name,
            'website': value_rec.website,
            'email': value_rec.email,
            'fields': opf,
            'object': value_rec,
            'options': options
        }
        html = self.pool["ir.ui.view"].render(cr, uid, "base.contact", val, engine='ir.qweb', context=context).decode('utf8')
        return HTMLSafe(html)
class QwebView(orm.AbstractModel):