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)


return self.html_url_format % self.html_url_args
    def stat(self):
        if not (self.inline or self._filename or self._ir_attach):
            path = filter(None, self.url.split('/'))
            self._filename = get_resource_path(*path)
            if self._filename:
                return
            try:
                
                fields = ['__last_update', 'datas', 'mimetype']
                domain = [('type', '=', 'binary'), ('url', '=', self.url)]
                ira = self.registry['ir.attachment']
                attach = ira.search_read(self.cr, openerp.SUPERUSER_ID, domain, fields, context=self.context)
                self._ir_attach = attach[0]
            except Exception:
                raise AssetNotFound("Could not find %s" % self.name)
    def to_html(self):