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)


# 
    # 
    def setUp(self):
        super(TestRelativeDatetime, self).setUp()
        # 
        self.registry('res.lang').load_lang(self.cr, self.uid, 'fr_FR')
    def test_basic(self):
        converter = self.get_converter('datetime', 'relative')
        t = datetime.datetime.utcnow() - datetime.timedelta(hours=1)
        result = converter(t, context={'lang': 'fr_FR'})
        self.assertEqual(result, u"il y a 1 heure")