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)


ira = self.registry['ir.attachment']
        domain = [('url', '=like', '/web/content/%%-%s/%s%s.%s' % (self.version, self.xmlid, ('%%' if inc is None else '.%s' % inc), type))]
        attachment_ids = ira.search(self.cr, openerp.SUPERUSER_ID, domain, order='name asc', context=self.context)
        return ira.browse(self.cr, openerp.SUPERUSER_ID, attachment_ids, context=self.context)
    def save_attachment(self, type, content, inc=None):
        ira = self.registry['ir.attachment']
        fname = '%s%s.%s' % (self.xmlid, ('' if inc is None else '.%s' % inc), type)
        values = {
            'name': "/web/content/%s" % type,
            'datas_fname': fname,
            'res_model': 'ir.ui.view',
            'res_id': False,
            'type': 'binary',
            'public': True,
            'datas': content.encode('utf8').encode('base64'),
        }
        attachment_id = ira.create(self.cr, openerp.SUPERUSER_ID, values, context=self.context)
        url = '/web/content/%s-%s/%s' % (attachment_id, self.version, fname)
        values = {
            'name': url,
            'url': url,
        }
        ira.write(self.cr, openerp.SUPERUSER_ID, attachment_id, values, context=self.context)
        if self.context.get('commit_assetsbundle') is True:
            self.cr.commit()
        self.clean_attachments(type)
        return ira.browse(self.cr, openerp.SUPERUSER_ID, attachment_id, context=self.context)
    def js(self):